feat: preserve track and passengers during route transitions

- Store pending track data and only apply when train resets to beginning
- Preserve passenger list display until entire train exits
- Prevent visual jumps by keeping old route data visible during fade-out
- Track transitions now seamless: old track/passengers persist until trainPosition < 0

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

apps/web/src/app/games/complement-race/components/RaceTrack/SteamTrainJourney.tsx

@@ -142,11 +142,65 @@ export function SteamTrainJourney({ momentum, trainPosition, pressure, elapsedTi
   // Get current route theme
   const routeTheme = getRouteTheme(state.currentRoute)
 
+  // Track previous route data to maintain visuals during transition
+  const previousRouteRef = useRef(state.currentRoute)
+  const [pendingTrackData, setPendingTrackData] = useState<ReturnType<typeof trackGenerator.generateTrack> | null>(null)
+
+  // Preserve passengers during route transition
+  const [displayPassengers, setDisplayPassengers] = useState(state.passengers)
+  const previousPassengersRef = useRef(state.passengers)
+
   // Generate track on mount and when route changes
   useEffect(() => {
     const track = trackGenerator.generateTrack(state.currentRoute)
-    setTrackData(track)
-  }, [trackGenerator, state.currentRoute])
+
+    // If we're in the middle of a route (position > 0), store as pending
+    // Only apply new track when position resets to beginning (< 0)
+    if (state.trainPosition > 0 && previousRouteRef.current !== state.currentRoute) {
+      setPendingTrackData(track)
+    } else {
+      setTrackData(track)
+      previousRouteRef.current = state.currentRoute
+      setPendingTrackData(null)
+    }
+  }, [trackGenerator, state.currentRoute, state.trainPosition])
+
+  // Apply pending track when train resets to beginning
+  useEffect(() => {
+    if (pendingTrackData && state.trainPosition < 0) {
+      setTrackData(pendingTrackData)
+      previousRouteRef.current = state.currentRoute
+      setPendingTrackData(null)
+    }
+  }, [pendingTrackData, state.trainPosition, state.currentRoute])
+
+  // Manage passenger display during route transitions
+  useEffect(() => {
+    // If we're starting a new route (position < 0) or passengers haven't changed, update immediately
+    if (state.trainPosition < 0 || state.passengers === previousPassengersRef.current) {
+      setDisplayPassengers(state.passengers)
+      previousPassengersRef.current = state.passengers
+    }
+    // Otherwise, if we're mid-route and passengers changed, keep showing old passengers
+    else if (state.trainPosition > 0 && state.passengers !== previousPassengersRef.current) {
+      // Keep displaying old passengers until train exits
+      // Don't update displayPassengers yet
+    }
+
+    // When train resets to beginning, switch to new passengers
+    if (state.trainPosition < 0 && state.passengers !== previousPassengersRef.current) {
+      setDisplayPassengers(state.passengers)
+      previousPassengersRef.current = state.passengers
+    }
+  }, [state.passengers, state.trainPosition])
+
+  // Update display passengers during gameplay (same route)
+  useEffect(() => {
+    // Only update if we're in the same route (not transitioning)
+    if (previousRouteRef.current === state.currentRoute && state.trainPosition >= 0 && state.trainPosition < 100) {
+      setDisplayPassengers(state.passengers)
+    }
+  }, [state.passengers, state.currentRoute, state.trainPosition])
 
   // Generate ties and rails when path is ready
   useEffect(() => {
@@ -372,13 +426,13 @@ export function SteamTrainJourney({ momentum, trainPosition, pressure, elapsedTi
 
   // Memoize filtered passenger lists to avoid recalculating on every render
   const boardedPassengers = useMemo(() =>
-    state.passengers.filter(p => p.isBoarded && !p.isDelivered),
-    [state.passengers]
+    displayPassengers.filter(p => p.isBoarded && !p.isDelivered),
+    [displayPassengers]
   )
 
   const nonDeliveredPassengers = useMemo(() =>
-    state.passengers.filter(p => !p.isDelivered),
-    [state.passengers]
+    displayPassengers.filter(p => !p.isDelivered),
+    [displayPassengers]
   )
 
   // Memoize ground texture circles to avoid recreating on every render

apps/web/src/app/games/complement-race/context/ComplementRaceContext.tsx

@@ -374,11 +374,11 @@ function gameReducer(state: GameState, action: GameAction): GameState {
         ...state,
         currentRoute: action.routeNumber,
         stations: action.stations,
-        trainPosition: 0,
+        trainPosition: -5, // Start off-screen to the left for smooth fade-in
         deliveredPassengers: 0,
         showRouteCelebration: false,
-        momentum: 0,
-        pressure: 0
+        momentum: 50, // Give some starting momentum for the new route
+        pressure: 50
       }
 
     case 'COMPLETE_ROUTE':