station-master/CHANGELOG.md

# Changelog

Detail behind each commit. Commit messages stay high level; the reasoning, the measurements, and
the things that turned out to be wrong live here.

Measured figures are 100 solitaire Standard games with the developer bot unless stated otherwise.
The target is 20 Revenue over 5 Days.

---

## Unreleased

### The replay behaves like the game it is replaying

Sound and the district auto-hide were built for the playable page and the replay had neither, which
made watching a game back a poorer experience than playing it. Both are there now, and both are the
SAME implementation rather than a second copy:

- `cuesFor` decides what happened — a Stage ended, a Day turned, a train was built — and is imported
  by the live game and called by the replay recorder, so the two cannot disagree about when a Stage
  ended. Frames carry their cues.
- `playCue` decides what that sounds like. It was three module-level functions with a shared
  `AudioContext`; it is now one self-contained function, embedded into the replay by `toString()`
  exactly as the board renderers are. The context is parked on `window` because the embedded copy
  has no module scope to keep it in.

**Sounds fire only when stepping FORWARD one frame.** Scrubbing across a hundred frames would
otherwise fire a hundred whistles at once, and stepping backwards would sound a Stage ending that is
being un-done.

Both default to the quiet, tidy setting: muted, and auto-hide on.

The replay's DOM stub had no `classList`, so the page threw while rendering rather than folding a
panel. It has one now — the same gap the playable page's stub had, and the same fix.

### Playtest fixes

**The end-of-line labels were cut to three letters.** "trains enter" and "trains leave" hang off the
ends of the route, and the canvas padding was sized for the buffer stops alone — so the west end read
"TER" and the east "tra". Padding now allows for the words.

**"End my turn" became "End Local Operations".** It ends the phase, and the old wording invited the
reading that it ended something smaller.

**The hand limit is a limit, not a toll on drawing.** Drawing a card set "you must now play one",
so a player who had already played two cards and drew back to three was still forced to spend one.
§6.2 is a hand LIMIT — "reduce his hand to no more than three cards", four with a Red Flag — and
that is now the only thing that blocks the end of a turn. It is derived from the hand at each
render, and a test asserts the page and the engine never disagree about whether the turn may end.
The engine removes `draw.end` outright when the hand is over the limit, so the page draws a disabled
button naming the reason rather than silently offering no way out.

**A train now says what its card calls for.** Reported on seed 22222: Extra X22 offered a caboose
and nothing else with no reason given. The rules were right — "Pee-Dee" is a per-diem train whose
consist is one caboose and no cars, and §8.2 forbids the wrong cars however few are carried — but
the screen never said so, which reads as a broken game. The New Train header now names the train and
its consist: *Making up Extra X22 "Pee-Dee": its card calls for 1 caboose — Per-diem train. May only
pick up MTs.*

**The auto-hide control said what the panel was doing, not what pressing it would do.** "auto ·
folded" reads as a status line and was missed; it now reads "auto-hide: on — click to keep open",
and looks like a control rather than a caption.

**The whistle is twice as long,** and sound now defaults to OFF.

### A legal move you could not click

Reported from playtesting: laying a straight offered three places in the list and highlighted one on
the board.

Empty squares were drawn by a separate `ghostSvg` and spliced into the SVG afterwards. It computed
its origin over cells PLUS the offered spots, while `officeSvg` sized itself over cells alone — so
the two disagreed the moment a legal square lay outside the played cards, which is every square that
would EXTEND the district. Reproduced on seed 555: three placements offered, the empty one drawn at
y=99 on a canvas 103 tall, i.e. off it entirely.

`officeSvg` now takes the ghosts and sizes itself over both, so there is one origin and one canvas,
and `ghostSvg` is deleted rather than fixed — a second coordinate system was the bug, not a detail
of it. It was also imported by the replay and never called there.

A test walks six seeds and asserts every square the menu offers has a target drawn inside the
canvas: 129 squares across 47 placeable subjects. It fails on the old code with the exact
coordinates above.

### The railroad, heard

A whistle at the end of each Stage, the grade-crossing bell at the end of each Day, and the
conductor when a train is built. **Off by default** — everything here is synthesised rather than
recorded, so it is a placeholder for real audio and a playtester who did not ask for noise should
not get any. One click in the title bar turns it on, and that click doubles as the gesture browsers
require before audio may start.

**Synthesised, not sampled**, and that is a constraint rather than a preference: the site is a static
folder that fetches nothing — there is a test asserting no page reaches an external host — so audio
would have to be committed to the repo, and a plausible whistle is not something to invent. A real
steam whistle is a CHORD of several chambers slightly out of tune with each other plus the breath of
the steam, which three detuned partials and a band of filtered noise get most of the way to. The
bell is inharmonic partials struck twice, which is what separates a bell from a beep.

**"All aboard" is speech, and speech cannot be faked with oscillators.** `speechSynthesis` is built
into the browser, needs no asset and works offline, so it says the words; where the platform has no
voice installed a two-note conductor's call takes its place rather than nothing happening.

The model names WHAT happened — a Stage ended, a Day turned, a train was made up — and the page
decides what that sounds like. A Day boundary rings the bell only: both would collide, and the bell
is the bigger event. The first Stage of the game announces nothing, because a Stage beginning is the
previous one ending and there is no previous one.

Counted against the clock rather than trusted: over a full game, 60 Stage boundaries produced 55
whistles and 5 bells, and 5 Days produced 5 bells.

### Regions, drawn from what the crossing already cost

§2.1 divides a Mainline card into two regions and §8.2 moves a train one region per Stage. The
engine had replaced that with `crossingStages()` — `implications.md` records it plainly: "The
`Region` model is gone" — because ten card types with real speeds cannot be expressed by one region
a Stage. Both `REGIONS_PER_MAINLINE_CARD = 2` and `entryPoints` survived as dead constants.

The map draws regions again without reinstating the mechanic, because the printed cards say how:
they carry **Start positions**, so a train with Brakemen *enters further along the card*. That is
the same fact as "takes a Stage off the crossing", in different coordinates. So position falls out
of what the engine already knows:

    entry = REGIONS - stagesTotal        position = clamp(entry + elapsed)

| crossing | drawn as | |
| --- | --- | --- |
| 1 Stage (a 60 card) | enters at region 2, gone next Stage | the printed Start position |
| 2 Stages (a 30 card) | region 1 → region 2 | **§8.2 exactly** |
| 3 Stages (slow train) | region 1 → region 1 → region 2 | fixed distance, slow train |

`entry` is deliberately allowed to go negative and only the final position is clamped: that keeps a
slow train's extra Stage at the START, where being slow shows. Clamping the entry instead parked it
at the exit, reading as a train that raced across and then waited — which is what the first version
did, and what the test now forbids.

`Transit` gains `stagesTotal`, set on entry. State, not rules: nothing reads it to decide anything,
so the bot and every balance figure are untouched, and a save is a seed plus intents so there is
nothing to migrate. It cannot be recomputed later — a modifier played onto the card mid-crossing
would change the answer and make the train jump backwards.

Division Points draw one region, the queue. Running Track cards inside a district draw none: a crew
moves there by Moves, not Stages, so it occupies a card outright.

### The Division map shows the whole route

The map drew one box per node, which collapsed each player's entire district into a single "Office"
tile — so the track a train actually runs along was invisible on the only view that shows where
trains are.

An Office now expands into its **Running Track, Limits to Limits**. That is the right cut rather than
a compromise: the through route IS the Running Track, and everything hanging beneath it is secondary
track a crossing train never touches. Sidings, industries and the load pipeline stay in the district
view, which is where they can be read.

    West DP · Mainline · [Limits … Office … Limits] · Mainline · [ … ] · Mainline · East DP

**Trains are shown wherever they are.** On a Running Track card, on a Mainline card, queued at a
Division Point. A crew working BELOW the Running Track has no position on the through route, so it
is reported against the district — "2 switching below" — rather than drawn somewhere it is not.
Projecting a siding onto the through line would be a lie the map cannot support.

**Seating.** Players sit around a table, so the route is laid out the way they do: one row alone,
two rows facing, a horseshoe of three, a square of four. The Division is a LINE and not a loop —
trains enter at one Division Point and leave at the other — so the shape is deliberately left open,
with buffer stops at both ends and the gap labelled "trains enter" and "trains leave". Closing it
into a ring would promise a connection the rules do not have.

Layout is geometry with no visual feedback loop, so it is tested rather than eyeballed: for 1, 2, 3
and 4 players no two cells may overlap and none may fall outside the canvas.

### The district folds itself away

Auto-focus. The district is worth its vertical space during the phases that change it — Local
Operations and Cargo — and not during New Train, Mainline or Supervisor Shift, where the Division
map is what matters. Folded, it leaves a summary line rather than vanishing, because a panel that
disappears entirely reads as broken.

A manual toggle overrides it and stays put. This is DISPLAY state and never game state: two players
at the same table may reasonably want it set differently.

### The replay was storing the same things over and over

Halved, near enough: **3415 KB → 1877 KB** on a 735-frame game.

The TODO said to carry `links` forward. Measured first, that would have bought 5% of the `cells`
payload — `cells` is 62% of the file, and inside it the cost is elsewhere:

    what       32% of cells   long prose, identical on every turnout in the district
    facility   24% of cells   the SAME object already serialised in the frame's `facilities`
    identity   26% of cells   row/col/kind/label/running/links, fixed once the card is laid

So all three are interned. A card's identity and its description are written once for the whole
recording and referenced by integer, and a cell points at its facility instead of carrying a second
copy of it. What stays per-frame is what genuinely changes: the cars standing there, the crew, and
any Enhancement laid on the card.

`rehydrateCells` is exported and emitted into the page by `toString()`, the same trick the two board
renderers use — a second copy inside the page's inline script could drift from the packing and the
symptom would be a board drawing the wrong cards rather than an error. The round-trip test runs that
exact function over every frame, resolving nulls the way the page does.

### Rolling stock was leaving the game

Four things were tried against bot revenue. **One of them was worth more than everything else in
this changelog combined, and it is the one that had been ranked third and predicted not to matter.**

**The Classification Yard was write-only.** Seven places pushed cars into it — retired trains,
collisions, unjams, set-outs — and nothing in the engine ever read it. Rolling stock drained one way
out of the game: 30 cars dead by the end of a game, **37% of the 80 dealt at setup**. Gap 2c says
engines and cabooses return to the Division Yard and everything else to Classification; the
recovered rules never say how Classification empties.

**ASSUMPTION, flagged rather than derived:** sorting cars for redistribution is what a
classification yard is for, and a Day is its natural cycle, so they now return at the Day boundary.
This is a rules decision that wants confirming against the source.

Paired over 400 seeds: **+2.32 ± 0.52, t = 8.79**, 194 seeds better against 44. Revenue 6.70 → 9.02.

Why it was mis-ranked is worth recording. The Division Yard does not run *dry* in five Days — 16.6
loaded freight cars remain, empty in 2 games of 100 — so it was reasoned that supply could not be
binding. The aggregate was never the point: what starves freight is not having the RIGHT commodity
at the moment a green box needs stocking, and returning classified cars keeps the mix alive.

### Three things that did not work, kept because the measurement is the result

Measured paired over the same 400 seeds, which is the only way to see effects this size.

**Capping the draw option: worth nothing.** 62% of all Local Operations actions went to drawing and
12.6 of 29 cards drawn were discarded, so a draw into a full hand converts straight into a discard.
Refusing it: **-0.10 ± 0.13**, and 379 of 400 seeds byte-identical. The branch almost never fires.

**Pairing the two halves of a load: worth nothing.** Sampling every outbound industry at every
loadUnload phase, 76% of the time it had NEITHER a stocked green box NOR a spotted car, and only 5%
of Stages had a single workable facility. Letting the bot switch for a stocked box without waiting
for a train at the Office changed nothing measurable — folded into the -0.10 above. The diagnosis
was right and the prescription did not address it: there is usually nothing to switch.

**Waking a dead branch made things worse.** `chooseLocalOption` tested
`options.some(i => i.type === 'mainline.modify')`, but that intent requires `s.turn.option === 'draw'`
and the test runs while the option is still null — `legalActions` had already filtered it out, so the
branch could never fire and never had. Rewriting it to check the hand cost **-0.64 ± 0.32
(t = -3.90)**, 104 seeds worse against 35. Its comment claimed the value compounds like a train
card's; it does not. The branch is now deleted, with the measurement in its place.

**The Enhancements other than Interlocking are worth exactly nothing** — and cost nothing either:
**-0.01 ± 0.41 (t = -0.04)** when the bot is forbidden to place any of them. Left as they are; there
is nothing to gain by restricting them. Note the first attempt at this experiment showed 0/400 seeds
changed, which was the experiment failing rather than the answer: a `card.play` fallback with no
placement filter was still placing them.

| | before | after |
| --- | --- | --- |
| revenue (200 games) | 6.7 | **8.7** |
| wins | 6.5% | **12%** |
| freight loads | 2.8 | 3.2 |
| cars dead in the Classification Yard | 28.4 | **~0** |

### A Running Track with nowhere to put an Enhancement

Every penalty in the game has one cause. Across 100 games, all 48 were `collision: no free A/D
track` — 2.70 revenue a game, **27% of gross**, concentrated in about a fifth of games and
responsible for the −47 tail.

The rules already answer it. Interlocking prints "may stop an inbound train on the Limit Track", and
`advance.ts:621` holds the train at the Limits instead of colliding. **It had never once been
placed.** Nor had any train ever been held at the Limits.

The reason was structural and nothing to do with Interlocking. The bot builds minimal two-arc
run-arounds — an `ne` arc meets an `nw` arc directly — so it never needed a straight and laid none:
**0.00 straights on the Running Track across 100 games.** Interlocking, Water Column and Telegraph
all require one; Yard Office and Small Yard want a Secondary Track Straight; Telephone and Radio
chain off Telegraph. Thirteen of the eighteen Enhancement cards that go on the board were
unplayable. They were drawn 3.78 a game and placed 0.64.

`bestTrackLay` now scores one straight onto the Running Track. Enhancements placed went 0.64 → 3.01
across nine types where only Overpass had ever appeared, and Interlocking now reaches the board in
about a quarter of games.

**On whether it pays — measured properly, and the answer is qualified.** Revenue per game has a
standard deviation of ~9, so a 100-game run carries roughly ±1.0 of noise. Run against the same 400
seeds, paired:

| | without | with |
| --- | --- | --- |
| revenue | 5.99 | **6.70** |
| collisions cost | 2.70 | **1.91** |
| worst single game | −47 | **−24** |
| most collisions in a game | 10 | 6 |
| wins | 5.3% | 6.5% |

Paired per-seed the change is **+0.70 ± 0.74 (95% CI), t = 1.87** — not significant on its own. And
154 seeds improved against **165 that got worse**: the mean gain comes from removing catastrophes,
not from making a typical game better. What justifies keeping it is that the mechanism is measured
directly and accounts for the whole effect — collision cost falls 0.79, and revenue rises 0.70.

**A correction to the numbers already in this file.** The per-100-game revenue figures reported for
the earlier changes carry the same ±1.0 noise, so the individual steps (5.0 → 6.0 → 6.5) were stated
more precisely than the sample supports. The cumulative move from 3.2 to ~6.7 is far larger than the
noise and stands; the individual increments should be read as indicative only.

### The bot was throwing away its own freight

**Routing turned out not to be the problem, which is why it was worth measuring first.** Of 1650
drops across 100 games, 940 (57%) landed on a facility that wanted the car and **zero** landed on
one that did not. The crew makes 9.4 correctly-targeted drops a game; the one-move-only destination
test costs nothing measurable. "70% of waiting loads have nothing spotted" was a misleading signal —
the cars arrive.

Following the freight from the other end found the leak. Loads were being **destroyed**:

    stockToOutbound   9.45/game
    loadStarted       2.71/game
    facilityUnjammed  3.10/game  from outbound  <- healthy waiting loads, discarded

`facilityUnjammed` from `outbound` splices the load out of the green box and pushes it to the
classification yard. That load cost a Local Operations action to stock, so discarding it is strictly
negative — and the bot did it more often than it started a load.

**Two fallbacks meeting, and the root cause is a familiar one: two rules for one act.**
`canStockProductively` decided the Freight Agent option was worth taking if a matching empty car was
spotted. The engine's `stockOutbound` additionally requires a **loaded car of that commodity in the
Division Yard** — which the predicate never checked. So the option was chosen believing a box could
be stocked when none could; the follow-through then found nothing stuck, nothing to clear and
nothing stockable, and fell through to "clear whatever is stuck" with nothing stuck.

Fixed by making the predicate ask the same question the engine does, by no longer using Freight
Agent as the idle default (switching at worst moves the crew toward the Office, which a train must
reach to depart at all, §8.1), and by ordering the last-resort unjam by what it costs to lose —
MEN|AT|WORK first, then the red box whose Revenue is already banked, and the green box last.

| | cars fixed | freight kept |
| --- | --- | --- |
| loads discarded from a green box | 3.10 | **0.00** |
| revenue | 6.0 | **6.5** |
| wins | 5/100 | **8/100** |
| trains scheduled | 3.0 | 3.4 |
| cards played | 16.6 | 19.3 |

**The gain is development, not freight.** Loads started held at 2.70 and freight revenue at 2.6 —
the recovered Local Operations actions went into drawing and switching rather than into the freight
chain. Green-box stocking fell 9.45 → 6.34 because the bot no longer stocks boxes it cannot serve.
The regression test asserts outbound unjams stay at zero *and* that genuine MEN|AT|WORK jams are
still cleared, so gutting the fallback would not pass it.

### Half the industries never asked for a car

Freight had not moved through two rounds of fixing the district, and this is why: **three of the six
industries were invisible when the bot chose what to put on a train.**

`wantedCars` consulted a hand-written `industry -> car type` switch that had drifted from the sheet.
It named `produceShed` and `oilRefinery` — neither is an industry — and omitted `freightHouse`,
`refinery` and `packingSheds`, which are. An industry it could not name returned null and was
skipped entirely, so it never requested a car. The Refinery is the only source of tank traffic, so
tank cars boarded a train 0.07 times a game and were **dropped by a crew zero times in 100 games**,
while 23 of 79 waiting loads sat at an industry that wanted one.

It now derives the commodities from `INDUSTRY_PROFILES`, which is the sheet. The switch is deleted
rather than corrected — a second copy of the mapping is the bug, not the values in it.

**A second, narrower collapse.** `facilityCarType` returned `carTypes[0]`, so the second commodity
of a two-commodity industry was unreachable: a Power Plant burns coal OR oil, a Grocer's Warehouse
receives dry goods OR perishables. `facilityCarTypes` (plural) now returns the full set, and the
bot's spotting and switching checks accept any of them.

Worth stating precisely: the engine's `WRONG_CAR_TYPE` gate was corrected to use the full set too,
but that changes nothing today — both two-commodity industries are inbound-only, so
`freightAgent.stockOutbound` rejects them before the commodity is examined. That fix is latent. The
measured gain is entirely the bot side.

| | facilities fixed | cars fixed |
| --- | --- | --- |
| revenue | 5.0 | **6.0** |
| wins | 1/100 | **5/100** |
| freight revenue | 1.3 | **2.6** |
| freight share of gross | 25% | **37%** |
| loads completed | 1.27 | **2.60** |
| tank cars dropped | **0.00** | **0.62** |
| reefers dropped | 0.04 | 0.67 |

Both regression tests were checked against the bug they guard: restoring the stale map fails the
tank assertion, and collapsing `carTypes` to its first entry fails the profile assertion.

**Still 70% of waiting loads have nothing spotted at all.** The commodity mix is right now; the
volume reaching the industry tracks is not. That is a routing question — which facility the crew
takes a car to — rather than a car-choice one.

### Putting the industries on the siding

The run-arounds were being built and the industries were somewhere else — facilities sitting on one
stayed at **0.00 a game** even at 91 districts in 100 with a closed loop. The cause was blunt:
facility placement was `options.find(i => i.placement !== undefined)`, the first legal square the
generator happened to list, unscored, while track laying had sixty lines of scoring beside it.

**Facilities are now scored**, on the thing that decides whether a crew can serve them at all: a
placement in line with a siding still being built becomes part of the loop itself (its own through
track is a segment), so the crew reaches it from either end and can pass its standing cars (§A.5).
Merely touching reachable track is worth less; the Running Track is a penalty, because a car left
standing there is hit by the next arrival (§11.2).

**A second bug surfaced immediately, and it is the interesting one.** Scoring facilities onto the
siding row sent run-arounds *down*, 91 games in 100 to 36 — the industry took the square and the
loop stopped closing around it. `runsAcross` tested the card's KIND ("a track straight running
east-west"), so an industry standing in the line read as a dead end and the run refused to extend
through it. It now asks the card's PORTS instead. A Facility carries its own rails (§11.2), and so
do the Office and a Limits sign; what matters is whether a port faces this way.

The reachability walk is now shared between track laying and facility placement rather than written
twice — two copies would eventually disagree about whether a district connects, which is the one
thing both decisions rest on.

| | before sidings | sidings fixed | facilities fixed |
| --- | --- | --- | --- |
| facilities on a run-around | 0.00 | 0.00 | **1.08** |
| games with a run-around | 0/100 | 91/100 | 71/100 |
| revenue | 3.2 | 4.1 | **5.0** |
| collisions | — | — | 0.4 (was 0.6) |
| track pieces spent | 19.5 | 16.1 | 13.6 |

Run-arounds fall from 91 to 71 because facilities now compete for the siding squares — which is the
trade being made deliberately: a loop with an industry on it is worth more than an empty one.

**Freight did not follow.** Loads completed 1.42 → 1.27 and freight's share of gross 31% → 25%; the
revenue gain is passengers and fewer collisions. Of facilities holding a load, 77% still have
nothing spotted at all. The industries are now reachable and the right cars still are not arriving —
which is the car-selection problem in TODO, untouched by any of this.

### The bot was building stubs, not sidings

**Measured first: 0 run-arounds in 100 games.** A run-around is the engine's own definition of a
useful siding (`track.ts`) — double-ended, both ends reaching the main, and §A.5's facing-point move
is impossible without one. Every district the bot built was dead-end stubs, 3.86 of them a game,
plus 2.89 cards below the main that reached nothing at all. Before trusting a zero the detector was
handed a run-around built on purpose and found it from both ends.

**Three bugs, all the same shape: scoring on local form without checking it reaches anything.**

- The +12 rule was commented "close the loop back up to the main: the run-around is complete" and
  only tested that a neighbour ran east-west — never that the card above had a south port to join.
  The siding terminated in an arc pointing north into empty space. It now requires a way up above,
  asked of the engine's `hasPort` so the Office counts too; `divergesSouth` had looked for a turnout
  and missed the one way down that is on every board. An arc's facing also decides what it meets —
  `nw` joins west, `ne` joins east — so closing from the wrong side connected nothing.
- The east-west extension had no stopping condition, so the run went on past the last column it
  could rejoin at, in 96 of 100 games. The loop then missed by one card.
- **`bestTrackLay` never declined.** This was the one that actually mattered, and the first two
  fixes barely moved the overshoot without it: the function returned its best-scoring option
  unconditionally, so once the useful squares were taken it kept laying track because track was
  legal. Bonuses are now tracked apart from the distance score, and a piece that earns none is not
  laid — the Stage falls through to playing a card instead.

Anchors also have to be reachable from the main now. A stranded east-west straight made both its
neighbours look like legal extensions, so a fragment joined to nothing grew in both directions.

| | before | after |
| --- | --- | --- |
| games with a run-around | **0/100** | **91/100** |
| track laid east of the last way up | 96/100 | **0/100** |
| track pieces spent | 19.5 | 16.1 |
| revenue | 3.2 | **4.1** |
| freight revenue | 2.6 | **3.8** |
| cars dropped | 11.6 | 20.9 |
| loads completed | 0.99 | 1.42 |

Two regression tests, both walking the district with the engine's own `exitsFrom` so they cannot
credit a connection §A.1 forbids: one asserts run-arounds get closed, the other that no track is
spent east of the last column with a way up.

**Still open.** Facilities sitting *on* a run-around: 0.00. The loops get built and the industries
are not on them, so the run-around is not yet paying for itself in freight — which is the next
thread, not a finished one.

### Freight, drawn where the work happens

**The load pipeline moved onto the card.** A load crosses green → `MEN | AT | WORK` → a spotted car,
and that journey *is* freight. It was drawn only in the side panel, so a Laborer action — the whole
of the freight game — changed nothing on the card the player was looking at. The squares now sit
under the track, which is where the printed cards put them and why they are printed at all. The
tooltip names the same thing in words: which square the load is on, and what the next Laborer action
does with it.

**A collision found while placing them.** `overpass` and `facingPointLocks` are placed `onCard`, so
they can land on a facility, and the enhancement label's baseline ran straight through the new
squares. The label moved into the gap between the crew tray and the pipeline; a test now asserts the
two do not overlap rather than trusting the two constants to stay apart.

### Teaching the bot to use a siding

**Nose coupling, which the rules had and the engine did not.** §A.3: "engines also have couplers on
the front end, so a train can pick cars up onto its nose". Coupling always appended to the back, so
which end cars landed on did not exist — and that is precisely what a run-around is for. Cars met
running FORWARD now couple in front, cars met BACKING couple behind, so the approach decides which
car is next off the tail:

    running forward -> reefer, boxcar, hopper   next off: hopper
    backing up      -> boxcar, hopper, reefer   next off: reefer

The bot prefers a run-around to setting a car down, since it keeps the car — but only when the drop
can actually follow. Without that guard it ran the loop for its own sake (8 a game, 63 Moves), which
the shuttling regression correctly failed.

**A serious bug found on the way.** The `carsCoupled` reducer cleared **every card in the Office
Area**, not the ones the crew ran over — its own comment said "every card along the path" while the
code iterated the whole grid. One coupling anywhere deleted every standing car and every industry
track in the district, so loads worked over several Stages vanished when a crew picked up an
unrelated boxcar somewhere else. The event now names the cards it lifted from.

**A test replaced rather than relaxed.** "Under 60 Moves a game" started failing. That threshold was
calibrated when the crew coupled ~0.4 cars a game; it now couples ~8 and drops ~11, and a run-around
is *supposed* to cost several Moves. Counting Moves was always a proxy for aimlessness, so the test
now measures the thing itself — Moves per drop-or-coupling — which still fails when the bot is made
to wander.

| | before sidings | now |
| --- | --- | --- |
| revenue | 3.9 | 3.2 |
| freight | 0.9 | 1.0 |
| drops per game | 3.3 | **11.1** |
| couplings per game | ~0.4 | **7.9** |

Switching is transformed; revenue is not. The crew now does real work — roughly 4 Moves per
productive act, which is what running a loop costs — but that work is not yet converting into
Revenue. Where it goes next is an open question rather than a known fix.

---

## Board rendering, three-page site, curve geometry

Drawing the board as track, splitting the site, tooltips, and the curve fix.

### Board rendering

Two renderers in `src/sim/board-svg.ts`, chosen because they fail in opposite places:

- **Office Area** — the district stays a map. Cards on a grid with the rails drawn edge to edge, so
  a join is rail meeting rail rather than two descriptions that happen to agree. The through rail
  sits at a constant height on every card, which is the alignment the printed cards use.
- **The Division** — not a map but a queue of sections with hard capacities, so it is a dispatcher's
  diagram: one line per track, `1 of 2 free` under each section, `no limit — trains queue` at the
  Division Points.

Both are **self-contained** — no imports, no module-level helpers — because the playable app imports
them normally while the replay is a single HTML file with an inline script that cannot import
anything, and embeds them via `Function.toString()`. One implementation either way; a second copy
would eventually draw a different board from the same state.

Rails come from the engine's own `connectionsFor`, now exported. A drawn rail cannot claim a
connection the rules do not have — visible in that **Modifier cards draw no rails at all**.

Capacity, confirmed from the rules and now shown: Division Points unlimited, most Mainline cards 1,
Double Track and Uncontrolled Siding 2, Offices 1/2/3/4 by tier.

### Three pages

`index.html` is now a splash with two doors. The game moved to `play.html`; `replays.html` is a
directory.

**A replay is a save**, and a save is `{seed, history}`. The engine is deterministic and runs in the
browser, so re-submitting the same moves rebuilds the position exactly — **18 KB instead of 3.4 MB**,
about 190× smaller, small enough to email. A save that would describe an impossible position cannot
be replayed at all, because every step goes back through `applyIntent`; the viewer stops and says so
rather than showing a board the rules could not produce.

Static hosting cannot list a directory, so `replays/manifest.json` is generated at build time from
whatever is in `public/replays/`, with each file validated first.

### Tooltips

Reference detail is read once and printing it costs the space the board and action list need. A
single delegated tooltip (`src/web/tooltip.ts`) now carries card effects, action reasoning, and
facility state. Not the native `title`: that waits a second, cannot be styled, and never appears for
keyboard users.

### Curve geometry — the significant fix

A curve was modelled as a through track **plus** a diverging leg, which is a turnout. Consequences:

- Curves were **topologically identical duplicates of turnouts** — same connections, no distinction
  beyond the sharp curve's 2-Move cost.
- Every diverging leg went south, so **no piece anywhere reached north** except an n-s straight,
  which connects n↔s and nothing else.
- Therefore a district could only ever be a **vertical column**: no siding, no parallel track, no
  run-around, no second turnout back to the main.

The printed cards (`docs/tracks.png`, rows 3–4) show a curve as a single arc from edge to edge with
no through track. A curve is now a **two-port arc**, rotatable to `ne`/`nw`/`se`/`sw`. A sharp curve
is geometrically identical and costs 2 Moves. Turnouts keep §A.1 unchanged — a two-port arc has no
third port, so the absent-edge rule cannot apply to it.

Verified through the engine, not the types: a crew leaves the main at a turnout, runs a siding
parallel, and rejoins at the far end.

### Measurements

| | revenue | freight | sidings built |
| --- | --- | --- | --- |
| before | 4.1 | 0.5 | impossible |
| geometry only | **3.9** | 0.9 | possible, not sought |
| bot builds sidings | 3.3 | **1.1** | **59/60 games** |

Freight more than doubled and the harness recorded **its first win**, but overall revenue is down
from geometry-alone and the bad tail worsened (−29 → −59).

Capping turnouts at one or two measured **worse** (revenue 2.5, freight 0.6) — more ways off the main
means more industries the crew can reach, and that beats a tidy Running Track. The uncapped version
stands, with that measurement recorded at the code.

**The bot builds sidings but does not exploit them.** It pays about 20 of its 26 track pieces for
them while its switching logic still sets out dead weight on a spur rather than planning a
run-around. That is the next piece of work and where the revenue should appear.

---

## Earlier commits

Recorded from memory of the work rather than written at the time; detail thins going back.

### `f00255c` — more fixes and tweaks

Card descriptions on every card in hand and every face-up slot, the three Local Operations options
explained, the objective and pace in the header, and rotations named in words rather than
"rotation 2". Build stamp added (version, git SHA, `-dirty` for an uncommitted tree) because nothing
tracked what was deployed. Extra X22 was **not** a bug — a per-diem train whose card calls for a
caboose and nothing else — but "loaded caboose" was.

### `d1f689d` — name the caboose, the train and the Running Track hazard

`maneuver.redFlags` was labelled "Red Flags on tray3": the earlier tray-id fix checked the history
log, and the action list was a surface it missed. An industry on the Running Track does not block
traffic (§11.2 gives it rails) but a car left standing there is hit by the next arrival (§10).

### `dd300ac` — fix caching, Limits placement, and explain the board better

The first deploy served a fresh `index.html` against a **cached** `main.js`, which threw
`missing element: target` and never started. Every module import now carries the build tag. The
Limits fix was half-done: the sign could move, but nothing forbade building past it, so one straight
at (0,2) produced `limits · Whistle Post · limits · straight · limits`.

### `2eca9de` — playable browser build

The solitaire game as a static site, proven to need no server: full games run with every Node global
replaced by a throwing stub. Train cards stopped accepting a board placement — seed 555 had offered
Extra X15 at six squares with six rotations, all identical.

### `160190d` — the bot's reasoning in the replay

Decision panel showing what the bot chose, why, and every option it passed over, with the reasons
reported by the bot itself rather than re-derived by the viewer.

### `d261ad7` — parking trains, freight deadlock, Whistle Post lock-in

Three faults found by measurement rather than failing tests. Trains parked because `destinationsFor`
computed reverse as `facing === 'e' ? 'w' : 'e'`, so a crew facing south reversed to east — a port a
north-south card does not have. Freight ran at a **3% load completion rate** because a load started
with no spotted car parks on WORK and locks the industry track, blocking the very car that would
clear it. Office density doubled after 25 of 100 games never drew a Depot and never escaped a
one-track Whistle Post.