diff --git a/ttd.h b/ttd.h
--- a/ttd.h
+++ b/ttd.h
@@ -236,6 +236,10 @@ typedef void ClickTileProc(uint tile);
 typedef void AnimateTileProc(uint tile);
 typedef void TileLoopProc(uint tile);
 typedef void ChangeTileOwnerProc(uint tile, byte old_player, byte new_player);
+/* Return value has bit 0x2 set, when the vehicle enters a station. Then,
+ * result << 8 contains the id of the station entered. If the return value has
+ * bit 0x8 set, the vehicle could not and did not enter the tile. Are there
+ * other bits that can be set? */
 typedef uint32 VehicleEnterTileProc(Vehicle *v, uint tile, int x, int y);
 typedef void VehicleLeaveTileProc(Vehicle *v, uint tile, int x, int y);