Bystander or driver - no difference. In both cases the wheel is in your hands. Just because the steering of the train is not IN the train but outside doesn't make you anything else than responsible.

You are the only one there, so you are driving the train, regardless of whether or not you agreed to it - heck, let's just assume you came legally out of tunnel and there's no time to stop, but you do have a button on board that switches tracks. Exact same problem, you're driving the train.