It can be very hard to hear a far-away engine that might move the train. If you were on the car, you might fall off, possibly under the wheels. You might find yourself unable to get off the moving train and worse yet, you might fall of the moving train. There are no sidewalks for you to move to if a train comes. The train does not generally blow the whistle at the bridge.
Tunnels are only wide enough for the train itself and a train can come at anytime.
You never know what might shoot out from under the wheels of the train. Trains are inspected regularly, but straps and chains holding down freight can come loose at any time. This game can quickly end in death.