v0.7.4 — Red Flags hold a train out of your Limits (Gitea#19)

"If played, asked FLAG EAST or FLAG WEST. That stops all trains from entering your
limits from that direction (i.e. Flag East holds westbound trains). You can do this
if you see a problem or wish to complete switching."

REPLACES the old rule outright, per Jesse's call. Red Flags used to be played on a
stopped train out on the Mainline and protected it from a rear-ender: offered 4,212
times and played 4 across 600 games, a mechanic nobody used, and ABS Signals already
does that job better. The flag is now planted on one side of your own district and
holds the next train arriving from that side.

SPENT ON THE TRAIN IT STOPS. One card, one train, so there is no lifting action to
build, nothing to forget, and a flag cannot quietly strangle the Division. The held
train loses one Mainline Phase and comes in on the next — it buys a Stage to clear
the lead, which is what "wish to complete switching" asks for.

PLAYABLE OUT OF PHASE, which is the other half of the issue: when an arrival would
certainly collide and the district's owner holds the card, the phase breaks in and
asks. Offered ONLY to somebody holding one — a prompt with a single button is not a
choice, and it would leak that a collision is coming. The danger is read from §8.3's
own two triggers rather than restated, so the prompt cannot offer a flag against a
collision that will not happen.

Built on the decision union Gitea#5 introduced: this adds a `redFlag` case and
nothing else structural.

THE BOT STILL NEVER PLAYS IT, AND I MEASURED RATHER THAN ASSUMED. It now takes the
out-of-phase prompt unconditionally — the engine has already established the danger,
so there is nothing left to judge — and over 200 solitaire games `redFlagsSet` fires
ZERO times. The prompt needs an arrival that would collide (0.14 per game, about one
game in seven) to coincide with holding the card from a three-card hand out of 121.
So the anomaly exemption in sim.test.ts stays, but its comment no longer claims the
bot is unwilling: it is measuring deck luck. What is left to fix is the half of the
card a human would use, planting a flag on purpose to buy switching time, and TODO.md
now says that instead of the old finding.

A BUG WORTH RECORDING, because the next interruption will meet it too: the flag was
originally taken down in a `reduce` case, which never fires for an event advance.ts
emits — the phase driver mutates state and then describes it. The flag stayed up and
held every train that came. test/events.test.ts's unreduced-event registry is what
makes that class of mistake visible, and `redFlagSpent` is on it deliberately now,
with the reasoning.

858 tests pass.

Closes #19

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01EAgJSmeV8zrMh55Mj85ESb

src/engine/advance.ts

@@ -974,13 +974,22 @@ function moveTrain(s: GameState, id: TrayId, tray: CrewTray, events: GameEvent[]
      * half-applied. Asking after the `transits` filter below cost the train its place on the card
      * and it was never seen again — the question was asked and the answer had nowhere to land.
      */
-    if (dest?.kind === 'office' && yardOfficeQuestion(s, id, tray, dest.seat, events) === 'ask') {
-      return 'needsClearance';
+    if (dest?.kind === 'office') {
+      /**
+       * §Q, RED FLAGS (Gitea#19) — asked and answered before the train leaves the card, for exactly
+       * the reason the Yard Office offer is (see below): `needsClearance` unwinds the phase.
+       *
+       * Order matters. A flag stops the train OUTSIDE the Limits, so it never reaches the point
+       * where the Yard Office would be offered — flagging is about keeping a train out altogether.
+       */
+      const flagged = redFlagStop(s, id, tray, dest, events);
+      if (flagged === 'ask') return 'needsClearance';
+      if (flagged === 'held') return 'held';
+
+      if (yardOfficeQuestion(s, id, tray, dest.seat, events) === 'ask') return 'needsClearance';
     }
 
     node.transits = node.transits.filter((t) => t.tray !== id);
-    // Red Flags protect a train while it is stopped here; once it rolls, the flags come in.
-    if (node.redFlagged) node.redFlagged = node.redFlagged.filter((t) => t !== id);
 
     if (!dest) return 'held';
 
@@ -1087,10 +1096,10 @@ function evaluateClearance(
     // from moving". Flagging is per-train rather than per-card, so it protects one specific train
     // where ABS Signals protects everything on the card.
     //
-    // Both of these read the card the OTHER train is standing on rather than the card being
-    // entered. They were the same card while this only looked one card ahead; across a Subdivision
-    // they are not, and the protection belongs where the train it protects actually is.
-    if (onNode?.kind === 'mainline' && (onNode.redFlagged ?? []).includes(other)) return 'blocked';
+    // Red Flags used to protect a stopped train here as well. Gitea#19 replaced that rule outright
+    // (Jesse, 2026-08-29): a flag is now planted on a district's Limits and holds trains coming from
+    // one direction, so it never applies out on the Mainline. ABS Signals is what protects a train
+    // standing on a Mainline card now, and it always did the job better.
 
     /**
      * ABS Signals — "this is played on a mainline card to prevent collisions. If a collision would
@@ -1147,6 +1156,103 @@ function evaluateClearance(
  * way the train is already facing. Reversing is a separate Move (§A.5), so a Yard Office that can
  * only be reached by backing up is correctly "not in one move".
  */
+/**
+ * The flag comes down as it stops the train — one card, one train (Gitea#19).
+ *
+ * MUTATES RATHER THAN EMITTING A REDUCED EVENT, because this is the phase driver: `advance.ts`
+ * changes state directly and then describes what it did, and roughly a third of the event types are
+ * never reduced at all (`README.md`, and `tally.ts` on the same asymmetry). A `redFlagSpent`
+ * reducer case looked right and never fired — the flag stayed up and held every train that came.
+ */
+function spendFlag(
+  office: Extract<DivisionNode, { kind: 'office' }>,
+  tray: CrewTray,
+  side: Direction,
+  events: GameEvent[],
+): 'held' {
+  delete office.redFlag;
+  events.push({ type: 'redFlagSpent', seat: office.seat, side, trainNumber: tray.trainNumber ?? 0 });
+  events.push({
+    type: 'trainHeld',
+    trainNumber: tray.trainNumber ?? 0,
+    reason: 'Red Flags — held short of the Limits',
+  });
+  return 'held';
+}
+
+/**
+ * §Q, RED FLAGS (Gitea#19) — does a flag stop this train, and should its owner be offered one?
+ *
+ * Two jobs, because they are the same moment seen twice: a flag already planted stops the train
+ * outright, and a train about to run into trouble is the cue to offer a flag to somebody holding
+ * the card. "You can play the card normally or out of phase, but only if you need it."
+ *
+ *   - `held`    — a flag was up on the side this train is coming from. It loses this Mainline
+ *                 Phase and the flag comes down with it: one card, one train (Jesse, 2026-08-29).
+ *   - `ask`     — entering would collide and the district's owner holds a Red Flags card.
+ *   - `proceed` — neither.
+ *
+ * WHICH SIDE. A train running WEST arrives from the east, so `FLAG EAST` is what holds it — which
+ * is the example the issue gives, and the reason the flag names a side rather than a heading.
+ */
+function redFlagStop(
+  s: GameState,
+  id: TrayId,
+  tray: CrewTray,
+  dest: Extract<DivisionNode, { kind: 'office' }>,
+  events: GameEvent[],
+): 'held' | 'ask' | 'proceed' {
+  const from: Direction = tray.direction === 'east' ? 'west' : 'east';
+
+  // The answer to a prompt already put. Consumed here so the driver cannot ask twice.
+  const answer = s.clock.decisionAnswer;
+  if (answer && answer.kind === 'redFlag' && answer.train === id) {
+    s.clock.decisionAnswer = null;
+    if (!answer.flag) return 'proceed';
+    // The card was spent planting the flag; it stops this train and comes down again at once.
+    return spendFlag(dest, tray, from, events);
+  }
+
+  if (dest.redFlag === from) return spendFlag(dest, tray, from, events);
+
+  /**
+   * "In actual cases of danger… if there is a train or cars on the track and there will be a
+   * collision, then you break in with a dialog." The two ways an arrival collides are §8.3's own:
+   * no free A/D track, and cars fouling the Running Track. Asked only of a player who can actually
+   * answer — offering a flag to somebody holding no card is a prompt with one button.
+   */
+  const owner = playerAtSeat(s, dest.seat);
+  const holdsFlag = (s.decks.hands.get(owner) ?? []).some((cid) => {
+    const c = s.cards.get(cid);
+    return c?.kind.kind === 'maneuver' && c.kind.key === 'redFlags';
+  });
+  if (!holdsFlag) return 'proceed';
+  if (!arrivalWouldCollide(s, id, tray, dest.seat)) return 'proceed';
+
+  s.clock.pendingDecision = { kind: 'redFlag', train: id, seat: dest.seat, from };
+  return 'ask';
+}
+
+/**
+ * Would this arrival collide? §8.3's two triggers, asked before the train commits.
+ *
+ * Deliberately a READ of the same conditions `arriveAtOffice` enforces rather than a second rule:
+ * if these two ever diverge, the prompt offers a flag against a collision that will not happen, or
+ * stays silent before one that will.
+ */
+function arrivalWouldCollide(s: GameState, id: TrayId, tray: CrewTray, seat: SeatIndex): boolean {
+  const area = areaAtSeat(s, seat);
+  const hasInterlocking = [...area.grid.values()].some((c) => c.enhancements.includes('interlocking'));
+  const full = area.adOccupancy.length >= officeProfile(area.tier).adTracks;
+  // Interlocking turns a full Office into a hold rather than a collision, so it is not danger.
+  if (full && !hasInterlocking) return true;
+
+  // A coach may legally stand at the Office while its engine switches (§A.4's carve-out), so it is
+  // not a hazard to the next arrival. Anything else on the Running Track is.
+  const officeCard = area.grid.get(coordKey(area.officeCoord));
+  return officeCard !== undefined && officeCard.standing.some((c) => c.type !== 'coach');
+}
+
 /**
  * §11 (Gitea#5) — should the district's owner be asked about the Yard Office, and is there
  * anything to ask about?
@@ -1396,7 +1502,6 @@ function collide(
       if (n.kind !== 'mainline') continue;
       n.transits = n.transits.filter((t) => t.tray !== id);
       if (n.holding) n.holding = n.holding.filter((t) => t !== id);
-      if (n.redFlagged) n.redFlagged = n.redFlagged.filter((t) => t !== id);
     }
     /**
      * AND OFF THE A/D TRACK, for exactly the same reason as the transit above.

src/engine/apply.ts

@@ -59,6 +59,7 @@ import {
   coordKey,
   cutTowards,
   decisionActor,
+  officeNodeFor,
   isOperationalRail,
   playerAtSeat,
   pooled,
@@ -803,6 +804,25 @@ export function check(s: GameState, player: PlayerIndex, i: Intent): RejectionCo
     return null;
   }
 
+  /**
+   * §Q (Gitea#19) — the Red Flag prompt, the third interruption of the Mainline Phase.
+   *
+   * Only ever raised for a player who holds the card, so `flag: true` can always be paid for; the
+   * card is checked again here because `check` is the authority and a hand can change between the
+   * prompt being raised and answered.
+   */
+  if (i.type === 'mainline.redFlag') {
+    if (s.clock.pendingDecision?.kind !== 'redFlag') return 'NO_RED_FLAG_PROMPT';
+    if (decisionActor(s) !== player) return 'NOT_YOUR_TURN';
+    if (!i.flag) return null;
+    const held = (s.decks.hands.get(player) ?? []).find((id) => {
+      const c = s.cards.get(id);
+      return c?.kind.kind === 'maneuver' && c.kind.key === 'redFlags';
+    });
+    if (!held) return 'NO_SUCH_CARD';
+    return null;
+  }
+
   /**
    * §11 (Gitea#5) — the Yard Office offer, the second interruption of the Mainline Phase.
    *
@@ -1024,14 +1044,9 @@ export function check(s: GameState, player: PlayerIndex, i: Intent): RejectionCo
       const card = s.cards.get(i.cardId);
       if (!card || !(s.decks.hands.get(player) ?? []).includes(i.cardId)) return 'NO_SUCH_CARD';
       if (card.kind.kind !== 'maneuver' || card.kind.key !== 'redFlags') return 'WRONG_INTENT';
-      const tray = s.trays.get(i.trayId);
-      if (!tray) return 'NO_SUCH_TRAY';
-      // "A STOPPED train is prevented from being hit" — it protects a train that is standing on a
-      // Mainline card, which is the only place a rear-ender can happen.
-      if (tray.position.at !== 'mainline') return 'NO_PLACEMENT';
-      const node = s.division.nodes[tray.position.index];
-      if (!node || node.kind !== 'mainline') return 'NO_PLACEMENT';
-      if ((node.redFlagged ?? []).includes(i.trayId)) return 'OPTION_ALREADY_CHOSEN';
+      // §Q (Gitea#19) — a flag goes on your OWN Limits. There is no target train to name and no
+      // placement to find: the district is yours, and the only question is which side.
+      if (officeNodeFor(s, seatOf(s, player))?.redFlag === i.side) return 'ALREADY_FLAGGED';
       return null;
     }
 
@@ -1828,10 +1843,25 @@ function execute(s: GameState, player: PlayerIndex, i: Intent): GameEvent[] {
       ];
     }
 
-    case 'maneuver.redFlags': {
-      const tray = s.trays.get(i.trayId)!;
-      const index = tray.position.at === 'mainline' ? tray.position.index : -1;
-      return [{ type: 'redFlagsSet', player, cardId: i.cardId, trayId: i.trayId, node: index }];
+    case 'maneuver.redFlags':
+      return [{ type: 'redFlagsSet', player, cardId: i.cardId, seat: seatOf(s, player), side: i.side }];
+
+    case 'mainline.redFlag': {
+      const pending = s.clock.pendingDecision;
+      const trainId = pending?.kind === 'redFlag' ? pending.train : '';
+      const seat = pending?.kind === 'redFlag' ? pending.seat : 0;
+      const side = pending?.kind === 'redFlag' ? pending.from : 'east';
+      if (!i.flag) return [{ type: 'redFlagRuled', player, trainId, flag: false }];
+      const cardId =
+        i.cardId ??
+        (s.decks.hands.get(player) ?? []).find((id) => {
+          const c = s.cards.get(id);
+          return c?.kind.kind === 'maneuver' && c.kind.key === 'redFlags';
+        })!;
+      return [
+        { type: 'redFlagsSet', player, cardId, seat, side },
+        { type: 'redFlagRuled', player, trainId, flag: true },
+      ];
     }
 
     case 'maneuver.flyingSwitch': {
@@ -2260,14 +2290,23 @@ export function reduce(s: GameState, e: GameEvent): void {
     }
 
     case 'redFlagsSet': {
-      const node = s.division.nodes[e.node];
-      if (node?.kind === 'mainline') {
-        node.redFlagged = [...(node.redFlagged ?? []), e.trayId];
-      }
+      const node = officeNodeFor(s, e.seat);
+      if (node) node.redFlag = e.side;
       spendCard(s, e.player, e.cardId);
       break;
     }
 
+    /**
+     * §Q (Gitea#19) — `redFlagSpent` is NOT reduced, deliberately. It is emitted only by the phase
+     * driver, which mutates state itself and then describes it (`advance.ts`'s `spendFlag`), so a
+     * case here would be dead code that reads as the live one.
+     */
+
+    case 'redFlagRuled':
+      s.clock.pendingDecision = null;
+      s.clock.decisionAnswer = { kind: 'redFlag', train: e.trainId, flag: e.flag };
+      break;
+
     case 'flyingSwitch': {
       const tray = s.trays.get(e.trayId);
       const area = areaOf(s, e.player);

src/engine/events.ts

@@ -87,7 +87,12 @@ export type GameEvent =
   /** `variant` is the chosen orientation (Gap 11); it must be replayable, so it rides the event. */
   | { type: 'cardPlayed'; player: PlayerIndex; cardId: CardId; placement?: GridCoord; variant?: number }
   | { type: 'mainlineModified'; player: PlayerIndex; cardId: CardId; node: number; key: string; became?: string }
-  | { type: 'redFlagsSet'; player: PlayerIndex; cardId: CardId; trayId: TrayId; node: number }
+  /** §Q (Gitea#19) — a flag planted on one side of a district's Limits. */
+  | { type: 'redFlagsSet'; player: PlayerIndex; cardId: CardId; seat: SeatIndex; side: Direction }
+  /** §Q (Gitea#19) — the flag stopped a train and came down with it. One card, one train. */
+  | { type: 'redFlagSpent'; seat: SeatIndex; side: Direction; trainNumber: number }
+  /** §Q (Gitea#19) — the district's owner answered the out-of-phase "flag against this train?". */
+  | { type: 'redFlagRuled'; player: PlayerIndex; trainId: TrayId; flag: boolean }
   | {
       type: 'trainsDestroyed';
       player: PlayerIndex;

src/engine/intents.ts

@@ -120,10 +120,24 @@ export type Intent =
    */
   | { type: 'mainline.modify'; cardId: CardId; node: number }
   /**
-   * Red Flags — protect a stopped train. The flagged train cannot be hit; an approaching train is
-   * held instead of colliding.
+   * §Q, RED FLAGS (Gitea#19) — plant a flag on one side of your own district.
+   *
+   * "If played, asked FLAG EAST or FLAG WEST. That stops all trains from entering your limits from
+   * that direction (i.e. Flag East holds westbound trains). You can do this if you see a problem or
+   * wish to complete switching."
+   *
+   * `side` names the side of the district the flag goes on, so a train arriving from that side is
+   * held. It REPLACES the old rule, which was played on a stopped train out on the Mainline and
+   * protected it from a rear-ender: measured at 4,212 offers and 4 plays across 600 games, a
+   * mechanic nobody used. ABS Signals already protects a train standing on a Mainline card.
    */
-  | { type: 'maneuver.redFlags'; cardId: CardId; trayId: TrayId }
+  | { type: 'maneuver.redFlags'; cardId: CardId; side: Direction }
+  /**
+   * The same card, played OUT OF PHASE at the moment of danger (Gitea#19) — "COLLISION RISK! FLAG
+   * AGAINST T2?". Answers a pending `redFlag` decision; `flag: false` declines and lets the
+   * collision happen. The side is not asked for: the train is already coming from one.
+   */
+  | { type: 'mainline.redFlag'; flag: boolean; cardId?: CardId }
   /**
    * Flying Switch — cut cars off behind the engine and roll them into an adjacent industry, without
    * the engine entering it.
@@ -312,7 +326,11 @@ export type RejectionCode =
   /** §3.3 (Gitea#11) — this seat has already voted on this extension. */
   | 'ALREADY_VOTED'
   /** §11 (Gitea#5) — answering a Yard Office offer that is not open. */
-  | 'NO_YARD_OFFICE_OFFER';
+  | 'NO_YARD_OFFICE_OFFER'
+  /** §Q (Gitea#19) — answering a Red Flag prompt that is not open. */
+  | 'NO_RED_FLAG_PROMPT'
+  /** §Q (Gitea#19) — this district already has a flag on that side. */
+  | 'ALREADY_FLAGGED';
 
 export type Rejection = { code: RejectionCode; message: string };
 

src/engine/legal.ts

@@ -95,6 +95,10 @@ function candidates(s: GameState, player: PlayerIndex): Intent[] {
     out.push({ type: 'mainline.yardOffice', take: true });
     out.push({ type: 'mainline.yardOffice', take: false });
   }
+  if (s.clock.pendingDecision?.kind === 'redFlag') {
+    out.push({ type: 'mainline.redFlag', flag: true });
+    out.push({ type: 'mainline.redFlag', flag: false });
+  }
 
   switch (s.clock.phase) {
     case 'localOps':
@@ -115,7 +119,8 @@ function candidates(s: GameState, player: PlayerIndex): Intent[] {
   for (const cardId of s.decks.hands.get(player) ?? []) {
     const k = s.cards.get(cardId)?.kind;
     if (k?.kind !== 'maneuver' || k.key !== 'redFlags') continue;
-    for (const [trayId] of s.trays) out.push({ type: 'maneuver.redFlags', cardId, trayId });
+    // §Q (Gitea#19) — a flag goes on one side of your own district, so the only choice is which.
+    for (const side of ['east', 'west'] as const) out.push({ type: 'maneuver.redFlags', cardId, side });
   }
 
   out.push({ type: 'redFlag.play' });

src/engine/state.ts

@@ -525,10 +525,23 @@ export type DivisionNode =
        * "Player sets orientation", so the direction is chosen when the card is placed.
        */
       gradeUp?: Direction;
-      /** Red Flags protecting a stopped train here, by tray. */
-      redFlagged?: TrayId[];
     }
-  | { kind: 'office'; seat: SeatIndex };
+  | {
+      kind: 'office';
+      seat: SeatIndex;
+      /**
+       * §Q, RED FLAGS (Gitea#19) — the side of this district a flag is planted on.
+       *
+       * "If played, asked FLAG EAST or FLAG WEST. That stops all trains from entering your limits
+       * from that direction (i.e. Flag East holds westbound trains)." So the value names the SIDE,
+       * and a train arriving from that side is held: a westbound train comes from the east.
+       *
+       * SPENT ON THE TRAIN IT STOPS (Jesse's ruling, 2026-08-29). One card, one train — the flag
+       * comes down as it is used, so there is no lifting action to build, nothing to forget, and a
+       * flag cannot quietly strangle the Division.
+       */
+      redFlag?: Direction;
+    };
 
 /** Ordered west to east. For N players: N Office nodes and N+1 Mainline cards. */
 export type Division = { nodes: DivisionNode[] };
@@ -610,7 +623,12 @@ export type PendingDecision =
    * §11 (Gitea#5) — an inbound freight may take the Yard Office instead of the Train Order Office.
    * Asked of whoever sits in `seat`, on the Mainline Phase the train arrives.
    */
-  | { kind: 'yardOffice'; train: TrayId; seat: SeatIndex };
+  | { kind: 'yardOffice'; train: TrayId; seat: SeatIndex }
+  /**
+   * §Q (Gitea#19) — a train is about to enter this district into a collision, and its owner holds a
+   * Red Flags card. "You can play the card normally or out of phase, but only if you need it."
+   */
+  | { kind: 'redFlag'; train: TrayId; seat: SeatIndex; from: Direction };
 
 /**
  * The answer, waiting to be consumed by the train that asked.
@@ -621,7 +639,8 @@ export type PendingDecision =
  */
 export type DecisionAnswer =
   | { kind: 'clearance'; train: TrayId; allow: boolean }
-  | { kind: 'yardOffice'; train: TrayId; take: boolean };
+  | { kind: 'yardOffice'; train: TrayId; take: boolean }
+  | { kind: 'redFlag'; train: TrayId; flag: boolean };
 
 export type Clock = {
   day: number;
@@ -1161,6 +1180,15 @@ export function playerAtSeat(state: GameState, seat: SeatIndex): PlayerIndex {
   return p;
 }
 
+/** This seat's node on the Division — where its Limits, and any Red Flag on them, live. */
+export function officeNodeFor(
+  state: GameState,
+  seat: SeatIndex,
+): Extract<DivisionNode, { kind: 'office' }> | null {
+  for (const n of state.division.nodes) if (n.kind === 'office' && n.seat === seat) return n;
+  return null;
+}
+
 /**
  * WHO MUST ANSWER the interruption, or null when nothing is pending.
  *

src/sim/bot.ts

@@ -170,7 +170,7 @@ export function makeDeveloperBot(tweaks: BotTweaks): BotPolicy {
     // Red Flags come before anything else — protection is only worth playing at the moment the
     // collision is actually pending, and that moment passes.
     const flags = worthFlagging(s, options);
-    if (flags) return because('a train of ours is stopped on a Mainline card with another train on it — Red Flags now or not at all', flags);
+    if (flags) return because('the engine says this arrival collides, and we hold a Red Flag — now or never', flags);
 
     // --- Load/Unload: spend every worker, then end. Each is a point, or a step toward one.
     //
@@ -1043,19 +1043,20 @@ function facilityWantsAt(
 }
 
 /**
- * Red Flags — "any time". Worth spending only when a train of ours is stopped out on the Mainline
- * with another train on the same card, which is the situation that becomes a rear-ender.
+ * §Q, RED FLAGS (Gitea#19) — spent only at the moment of danger.
+ *
+ * The card was redefined: it plants a directional flag on your own Limits rather than protecting a
+ * stopped train out on the Mainline, so the old heuristic ("is a train of ours sharing a Mainline
+ * card") no longer describes anything the card does.
+ *
+ * The bot now flags ONLY through the out-of-phase prompt, which the engine raises exactly when an
+ * arrival would collide (`redFlagStop`). That is a better policy than the old one and a much
+ * simpler one: the engine has already established the danger, so there is nothing for the bot to
+ * judge. It never plants a flag speculatively — it cannot tell whether it wants time to switch, and
+ * a flag spent early is a flag not there when a train is actually bearing down.
  */
-function worthFlagging(s: GameState, options: Intent[]): Intent | null {
-  for (const i of options) {
-    if (i.type !== 'maneuver.redFlags') continue;
-    const tray = s.trays.get(i.trayId);
-    if (!tray || tray.position.at !== 'mainline') continue;
-    const node = s.division.nodes[tray.position.index];
-    if (node?.kind !== 'mainline') continue;
-    if (node.transits.length > 1) return i;
-  }
-  return null;
+function worthFlagging(_s: GameState, options: Intent[]): Intent | null {
+  return options.find((i) => i.type === 'mainline.redFlag' && i.flag === true) ?? null;
 }
 
 /** A one-line account of which Load/Unload action was taken, and why it ranked first. */

src/sim/narrate.ts

@@ -218,10 +218,19 @@ export function narrate(e: GameEvent, ctx: NarrateContext = {}): Narration {
           ? `Realignment: Mainline card ${e.node} converted to ${e.became}`
           : `Played ${e.key} on Mainline card ${e.node}`,
       };
+    case 'redFlagSpent':
+      return {
+        tone: 'good',
+        text: `RED FLAG — Train ${e.trainNumber} stopped short of the ${e.side === 'east' ? 'Eastern' : 'Western'} Limits. The flag comes down with it.`,
+      };
+    case 'redFlagRuled':
+      return e.flag
+        ? { tone: 'plain', text: `Player ${e.player} flagged the approaching train` }
+        : { tone: 'plain', text: `Player ${e.player} waved the train through` };
     case 'redFlagsSet':
       return {
         tone: 'good',
-        text: `Red Flags set out to protect train ${e.trayId} on Mainline card ${e.node} — an approaching train must stop`,
+        text: `RED FLAGS set out on the ${e.side === 'east' ? 'Eastern' : 'Western'} Limits — the next train from that way is held short`,
       };
     case 'flyingSwitch':
       return {

src/sim/view.ts

@@ -1043,7 +1043,15 @@ export function describeIntent(s: GameState, i: Intent): string {
       return `${cardName(s, i.cardId)} on ${shortWhere} — ${where}${effect ? `; ${effect}` : ''}`;
     }
     case 'maneuver.redFlags':
-      return `set Red Flags to protect ${trainName(s, i.trayId)} — an approaching train must stop short`;
+      return (
+        `FLAG ${i.side === 'east' ? 'EAST' : 'WEST'} — hold the next ${i.side === 'east' ? 'westbound' : 'eastbound'} ` +
+        'train short of your Limits, so you can finish switching'
+      );
+    // §Q, the out-of-phase play (Gitea#19) — "COLLISION RISK! FLAG AGAINST T2?"
+    case 'mainline.redFlag':
+      return i.flag
+        ? 'FLAG IT — stop the train short of your Limits, spending a Red Flags card'
+        : 'wave it through — let it come in';
     /**
      * §11, the Yard Office (Gitea#5). The offer interrupts the Mainline Phase, so the label has to
      * carry the whole question — there is no surrounding context on screen to lean on, and the
@@ -1109,7 +1117,15 @@ export function describeIntent(s: GameState, i: Intent): string {
     case 'redFlag.play':
       return 'play your red flag';
     case 'maneuver.redFlags':
-      return `set Red Flags to protect ${trainName(s, i.trayId)} — an approaching train must stop short`;
+      return (
+        `FLAG ${i.side === 'east' ? 'EAST' : 'WEST'} — hold the next ${i.side === 'east' ? 'westbound' : 'eastbound'} ` +
+        'train short of your Limits, so you can finish switching'
+      );
+    // §Q, the out-of-phase play (Gitea#19) — "COLLISION RISK! FLAG AGAINST T2?"
+    case 'mainline.redFlag':
+      return i.flag
+        ? 'FLAG IT — stop the train short of your Limits, spending a Red Flags card'
+        : 'wave it through — let it come in';
     /**
      * §11, the Yard Office (Gitea#5). The offer interrupts the Mainline Phase, so the label has to
      * carry the whole question — there is no surrounding context on screen to lean on, and the