Some high-speed trains wind up with a kind of air-cushion in front of them that can disrupt small objects like a person. This can make the effects unpredictable. But to answer your question if death by train is painful, the answer is nequivocally yes. Unless you know the exact type of locomotive, you can't predict anything. So, in all likelihood, given a decent setup, a train will kill you. But, it will probably hurt and not be over quick.
However, you do need to consider the effects you have on the train operator. It's one thing to CTB. It's a lot different to CTB and bring someone else who didn't ask into your method.