Index: src/train_cmd.cpp
===================================================================
--- src/train_cmd.cpp	(revision 8089)
+++ src/train_cmd.cpp	(working copy)
@@ -2934,6 +2934,20 @@
 
 	u = v;
 	BEGIN_ENUM_WAGONS(v)
+		/* When a vehicle crashes the presence on the track must be removed */
+		/* If a train collides on a bridge the other end of the bridge must be cleaned up as well */
+		if (IsTileType(v->tile, MP_TUNNELBRIDGE)) {
+			ClearTrackMask(v->tile);
+			if (IsBridge(v->tile)) {
+				DEBUG(misc, 2, "Crashed on bridge, tile=%d", v->tile);
+				ClearTrackMask(GetOtherBridgeEnd(v->tile));
+			} else if (IsTunnel(v->tile)) {
+				DEBUG(misc, 2, "Crashed in tunnel, tile=%d", v->tile);
+				ClearTrackMask(GetOtherTunnelEnd(v->tile));
+			}
+		} else {
+			RemoveTrackMask(v->tile);
+		}
 		v->vehstatus |= VS_CRASHED;
 	END_ENUM_WAGONS(v)
 
@@ -3018,6 +3032,37 @@
 	return NULL;
 }
 
+/**
+ * Set presence on track if the front engine enters or the last wagon leaves a tile.
+ */
+static void UpdateTrainTileCache(Vehicle *u, GetNewVehiclePosResult gp, TrackBits next_track)
+{
+	/* Only the last wagon may clear a tile */
+	if (u->next == NULL) {
+		RemoveTrackMask(gp.old_tile);
+		/* If on a bridge or in a tunnel, the other end must be freed as well */
+		if (IsTileType(gp.old_tile, MP_TUNNELBRIDGE)) {
+			if (IsBridge(gp.old_tile) && (!MayHaveBridgeAbove(gp.new_tile) || (MayHaveBridgeAbove(gp.new_tile) && !IsBridgeAbove(gp.new_tile)))) {
+				RemoveTrackMask(GetOtherBridgeEnd(gp.old_tile));
+			} else if (IsTunnel(gp.old_tile) && !IsTunnelInWay(gp.new_tile, u->z_height)) {
+				RemoveTrackMask(GetOtherTunnelEnd(gp.old_tile));
+			}
+		}
+	}
+	/* Only the first may trigger it */
+	if (GetPrevVehicleInChain(u) == NULL) {
+		SetTrackMask(gp.new_tile);
+		/* When entering a bridge or tunnel the other end must be activated as well */
+		if (IsTileType(gp.new_tile, MP_TUNNELBRIDGE)) {
+			if (IsBridge(gp.new_tile)) {
+				SetTrackMask(GetOtherBridgeEnd(gp.new_tile));
+			} else {
+				SetTrackMask(GetOtherTunnelEnd(gp.new_tile));
+			}
+		}
+	}
+}
+
 static void TrainController(Vehicle *v, bool update_image)
 {
 	Vehicle *prev;
@@ -3126,6 +3171,9 @@
 					chosen_dir = (Direction)b[2];
 				}
 
+				/* Keep the collision check updated with information */
+				UpdateTrainTileCache(v, gp, chosen_track);
+
 				/* Call the landscape function and tell it that the vehicle entered the tile */
 				r = VehicleEnterTile(v, gp.new_tile, gp.x, gp.y);
 				if (r & 0x8) {
@@ -3536,7 +3584,9 @@
 
 		do {
 			TrainController(v, true);
-			CheckTrainCollision(v);
+			if (GetTrackOverload(v->tile)) {
+				CheckTrainCollision(v);
+			}
 			if (v->cur_speed <= 0x100)
 				break;
 		} while (--j != 0);
Index: src/rail_map.h
===================================================================
--- src/rail_map.h	(revision 8089)
+++ src/rail_map.h	(working copy)
@@ -6,8 +6,8 @@
 #include "direction.h"
 #include "rail.h"
 #include "tile.h"
+#include "debug.h"
 
-
 typedef enum RailTileType {
 	RAIL_TILE_NORMAL         = 0x0,
 	RAIL_TILE_SIGNALS        = 0x40,
@@ -338,5 +338,63 @@
 	_m[t].m4 = 0;
 	_m[t].m5 = RAIL_TILE_DEPOT_WAYPOINT | RAIL_SUBTYPE_WAYPOINT | a;
 }
+/**
+ * Make trains aware to watch out for collisions
+ */
+static inline void SetTrackOverload(TileIndex t)
+{
+	DEBUG(misc, 2, "Turning on collision warning for tile %d", t);
+	SB(_m[t].m6, 3, 1, 1);
+}
 
+/**
+ * Make trains aware not to watch out for collisions anymore
+ */
+static inline void EndTrackOverload(TileIndex t)
+{
+	DEBUG(misc, 2, "Turning off collision warning for tile %d", t);
+	SB(_m[t].m6, 3, 1, 0);
+}
+
+/**
+ * Find out if collision check is needed
+ */
+static inline bool GetTrackOverload(TileIndex t)
+{
+	/* Check if track is overloaded/possible collision chance */
+	return GB(_m[t].m6, 3, 1);
+}
+
+/**
+ * Set the presence of trains in the map, if needed trigger an collision warning
+ */
+static inline void SetTrackMask(TileIndex t)
+{
+	if (GB(_m[t].m6, 2, 1)) {
+		SetTrackOverload(t);
+	}
+	/* This is not perfect, but should decrease load, this will still check if two trains are on the same tile */
+	SB(_m[t].m6, 2, 1, 1);
+}
+
+/**
+ * Remove the presence of a train from the map, remove collision warning when active
+ */
+static inline void RemoveTrackMask(TileIndex t)
+{
+	if (GetTrackOverload(t)) {
+		EndTrackOverload(t);
+	}
+	SB(_m[t].m6, 2, 1, 0);
+}
+
+/**
+ * Remove the presence of all trains a tile, remove collision warning as well, needed for bridges
+ */
+static inline void ClearTrackMask(TileIndex t)
+{
+	EndTrackOverload(t);
+	SB(_m[t].m6, 2, 1, 0);
+}
+
 #endif /* RAIL_MAP_H */