user card Hand becomes the action surface, the board shows the train, and a reverse stops turning it around, other bug fixes too.

src/engine/apply.ts

@@ -719,7 +719,20 @@ function execute(s: GameState, player: PlayerIndex, i: Intent): GameEvent[] {
           from,
           to: i.to,
           movesRemaining: s.turn.movesRemaining - 1,
-          facing: opposite(dest.entry),
+          /**
+           * A TRAIN THAT BACKS UP HAS NOT TURNED AROUND.
+           *
+           * `facing` is which way the ENGINE points, and this set it to the direction of travel on
+           * every move — so one reverse move silently spun the train about. Everything then read
+           * "forward" again, and a run-around became pointless: you could change ends for free by
+           * backing up twice.
+           *
+           * Running forward the engine leads, so it points the way the train went: `opposite(entry)`.
+           * Backing up it trails, still pointing the way it came, which is the port it arrived
+           * through. Both hold around a curve, where the compass heading changes but the engine's
+           * relationship to its train does not.
+           */
+          facing: i.reverse ? dest.entry : opposite(dest.entry),
         },
       ];
       if (dest.couples.length > 0) {

src/engine/legal.ts

@@ -13,6 +13,7 @@
  */
 
 import type { CarType, Hand, TrackGeometry } from './content.ts';
+import { enhancementRule } from './content.ts';
 import { check, areaOf, destinationsFor } from './apply.ts';
 import type { Intent } from './intents.ts';
 import type { GameState, GridCoord, PlayerIndex } from './state.ts';
@@ -140,8 +141,19 @@ function localOpsCandidates(s: GameState, player: PlayerIndex): Intent[] {
   for (const cardId of s.decks.hands.get(player) ?? []) {
     out.push({ type: 'card.play', cardId });
     const kind = s.cards.get(cardId)?.kind;
-    // Enhancements ATTACH to a card already down, so their candidates are the occupied cells.
-    const targets = kind?.kind === 'enhancement' ? attachments : placements;
+    /**
+     * Enhancements ATTACH to a card already down, so their candidates are the occupied cells — with
+     * one exception. ABS Signals goes on a MAINLINE card, and `checkEnhancementPlacement` reads
+     * `placement.col` as a Division node index for it. Offering it grid squares meant the engine
+     * accepted "(0, 1)" because node 1 happened to be a Mainline card: the label said one thing and
+     * the placement meant another, so the card appeared to be playable all over the Office Area.
+     */
+    const onMainline = kind?.kind === 'enhancement' && enhancementRule(kind.key)?.placement === 'mainlineCard';
+    const targets = onMainline
+      ? s.division.nodes.map((_, node) => ({ row: -1, col: node }))
+      : kind?.kind === 'enhancement'
+        ? attachments
+        : placements;
     // Orientation is chosen on placement, and a printed card turns but never flips, so the widest
     // variant set is TWO. Anything that is not track has a single orientation and needs one entry.
     const rotations = kind?.kind === 'track' ? variantsFor(kind.geometry, kind.hand).length : 1;