Not sure if it's the same case for you guys as it is in GTA3:MTA, but if trains cause a crash, it's because they are treated as a vehicle without collisions, so if it's not synced and a user enters it, it crashes everyone else because to them something triggered a non-collision object to be entered by a player, just like if you try to enter the non-collision Dodo that flies in the sky. If you somehow manage to spawn it with hacks and enter it, it crashes your game for the same reason.