v0.4.7 — the switching game: track order, the cut on your own card, and four rules

Eight play reports and one design that had been written up and not built. The
through-line is switching: what a card can hold, which end of a train a cut comes
off, which way a train meets cars standing on the line, and what the board and the
log say about all of it.

TRACK ORDER FOR STANDING CARS, AND THE CUT ON YOUR OWN CARD

Two reports turned out to be one root cause. `TrackCard.standing` claimed "in track
order (§A.3)" and had no defined orientation at all, while `CrewTray.consist` does
(nose first, relative to facing) — so every transfer between them was a conversion
nothing performed. §A.3 says what it should be: cars occupy the track "in the same
order they originally held, left-to-right". Left-to-right is west-to-east, and that
is now the defined orientation of `standing` and of an industry track through
`carsOn`. It is the board's orientation, not the train's, so it does not change when
a different train touches the card.

  - Setting out is batch-invariant. Four cars at once, four singles and two pairs
    parked three different orders, one of them physically impossible. Successive
    cuts off the same end stack up towards the engine, so the insertion point is the
    train's own place in the row.
  - Approaching a cut from either end now mirrors. `couples` is built nearest-first
    along the direction of travel and reverses onto the nose, so the farthest car met
    ends up nose-most — which is what makes a run-around worth its Move.
  - A train no longer drives through its own cut. The walk began at the neighbour of
    the start square and never read the start card, so a crew could set cars out and
    pull straight away from them. Coupling is mandatory (§A.4) and your own square is
    no exception; the cut counts against the four-car limit. Setting out off the end
    you are not leaving by still works.

`CrewTray.standingWest` records where a train stands among the cars on its card — a
train may set out off both ends on one square, so which side a cut is on is not
recoverable from the array alone.

On the board, the cut is drawn split at the train — west cars left, east cars right,
engine in the gap — and each car's tooltip says whether it stands ahead of or behind
the engine. The history says which end a cut came off, and a move's button separates
"takes your own boxcar back off this card" from cars found standing on the line.

Decided: taking your own cut back on the square you are standing on is UNDOING the
drop. It is exempt from trains 3/4's per-location freight budget, X13's "drop but not
pick up" and X22's "empties only", and it refunds the budget the drop spent.
Otherwise a legal-looking drop becomes silently one-way.

Measured, 200 paired seeds, developer bot: -0.55 revenue (t = -3.63), freight revenue
1.11 -> 0.56. That cost is the bot's, not the rule's — its trains run engine-first,
so at a stub industry it sets a car out between itself and the only way out, and the
correct play is §A.5's facing-point move, which is the cross-turn planning TODO.md
already records as out of reach of any bot. Filtering self-recoupling moves out of its
options took recoupling from 625 of 1,029 set-outs to 101 of 677, and all 101 that
remain are that case. Read the number as a bot measurement, not a balance one.

THE SUPERINTENDENT'S RULING NAMES THE TRAINS IT IS ABOUT

Reported: the Superintendent could not tell which train he was clearing. The heading
asks the question now — "may Train 6 follow Train 4 onto the same Mainline card?" —
and the trains moved to the FRONT of each button, because the button splits its label
at the first em-dash and showed only the head.

AN INDUSTRY TRACK HOLDS FOUR CARS, LIKE EVERY OTHER CARD

Reported at undo 188: "we wanted to drop two cars, but were only allowed to drop one."
An industry track was built as long as its box count, so a one-box industry had room
for one car. Box count is how much WORK an industry can hold, not how much RAIL it
has. Ordinary track was the other exception, unbounded; both are gone and every card
holds four.

THE FREIGHT AGENT MAY STAGE A LOAD BEFORE THE CAR IS THERE

§6.3 asks nothing of the industry track — the empty car belongs to §9.3's Load the
car, which is the Laborer's action. The gate now lives only there, so cargo can wait
on the dock while the car to ship it in is still being switched in. Nothing can jam:
a load in a green box is waiting, not stuck.

THE TRUCK DOCK UNLOADS, AND BRINGS NOBODY

+1 inbound, no Laborer. It printed +1 outbound and +1 Laborer, which made it a
longer-host-list copy of Forklifts. Beside Packing Sheds it now does nothing at all,
and the hand tooltip says so before it is played.

Also in this release, from the days before: Mainline card tooltips computed from the
crossing rule; an Extra starts from the Division Point its number sends it to; a
modifier's suppressed grant comes back when a Whistle Post is upgraded; the Oil
Refinery and the Grocer's Warehouse ship as well as receive, per the card reference;
and the dormant defences name the attack they answer. `.claude/` is now gitignored —
it holds Claude Code's worktrees, i.e. a second checkout of this repository.

570 tests, typecheck clean. The three published replays were re-recorded twice —
legality changed, so bot play changed. Full detail in CHANGELOG.md.

src/engine/track.ts

@@ -34,7 +34,7 @@ import type {
   TrackArc,
   TurnoutOrientation,
 } from './state.ts';
-import { carsOn, coordKey, isLockedByWork, isOperationalRail, spaceOn } from './state.ts';
+import { carsOn, coordKey, cutTowards, isLockedByWork, isOperationalRail, spaceOn } from './state.ts';
 
 // ---------------------------------------------------------------------------
 // Ports and geometry
@@ -316,6 +316,15 @@ export type MoveContext = {
   consistSize: number;
   /** The tray making the move, so it does not block itself. */
   self: TrayId;
+  /**
+   * How many cars on the START card stand west of this train (`CrewTray.standingWest`).
+   *
+   * The walk begins at `neighbour(start, initialExit)` and never looked at the start card at all, so
+   * a train drove straight through the cut it had just set out — in both directions, and without
+   * counting it against the four-car limit. Coupling is mandatory (§A.4) and a cut on your own
+   * square is no exception: pulling out through the end it sits at picks it up again.
+   */
+  standingWest?: number;
 };
 
 function cardAt(area: OfficeArea, c: GridCoord): TrackCard | undefined {
@@ -406,8 +415,20 @@ export function exploreMoves(
     if (firstCard) block(first, 'noJoin', 'the rails do not meet — two cards touching is not a join, and on a north or south edge both 45° legs must lie on the same diagonal');
     return { destinations: results, blocked };
   }
+  /**
+   * THE CUT ON YOUR OWN CARD, which the walk used to ignore entirely.
+   *
+   * Reported from play: "if I put cars off the nose on a given track, and my next move is go
+   * forward, I need to couple those cars right back on". Exactly so — and the engine let the train
+   * drive away and leave them. `cutTowards` takes only the cars between the engine and the end it is
+   * leaving by, so setting out off the nose and then BACKING away is still free, which is the whole
+   * point of setting out off one particular end.
+   *
+   * Ordered nearest-first like every other card's, so it simply seeds the accumulator.
+   */
+  const ownCut = cutTowards({ standingWest: ctx.standingWest }, carsOn(startCard), initialExit);
   const queue: Frontier[] = [
-    { coord: first, entry: opposite(initialExit), path: [], couples: [] },
+    { coord: first, entry: opposite(initialExit), path: [], couples: ownCut },
   ];
 
   while (queue.length > 0) {
@@ -443,14 +464,27 @@ export function exploreMoves(
       continue;
     }
 
-    // Mandatory coupling. Rejecting rather than truncating is deliberate: a move that would
-    // overfill the tray is illegal, not a move that picks up fewer cars.
-    const couples = [...node.couples, ...carsOn(card)];
+    /**
+     * Mandatory coupling. Rejecting rather than truncating is deliberate: a move that would
+     * overfill the tray is illegal, not a move that picks up fewer cars.
+     *
+     * NEAREST FIRST ALONG THE DIRECTION OF TRAVEL. `carsOn` runs west to east, so a train entering
+     * through the card's EAST port meets them back to front and the row has to be reversed. Without
+     * this the same parked cut produced an identical consist whichever way it was approached, when
+     * the two must mirror — which is the difference between a run-around being worth a Move and
+     * being pointless.
+     */
+    const met = node.entry === 'e' ? [...carsOn(card)].reverse() : carsOn(card);
+    const couples = [...node.couples, ...met];
     if (ctx.consistSize + couples.length > MAX_CONSIST) {
+      const own =
+        ownCut.length > 0
+          ? `, ${ownCut.length} of them the cut standing at the end of your OWN card that this move pulls out through`
+          : '';
       block(
         node.coord,
         'tooManyCars',
-        `too many cars — running here couples ${couples.length} standing car(s) onto a train already holding ${ctx.consistSize}, and ${MAX_CONSIST} is the limit. Coupling is mandatory: you may not run past a car and leave it (§A.4)`,
+        `too many cars — running here couples ${couples.length} standing car(s)${own} onto a train already holding ${ctx.consistSize}, and ${MAX_CONSIST} is the limit. Coupling is mandatory: you may not run past a car and leave it (§A.4)`,
       );
       continue;
     }