It's not accurate to say something going that slow does or not have that much force. It has momentum. Force is change in momentum, which happens over time. Something moving very slow but is very massive will apply a very small impulse (change in momentum) to a human because the collision with the human is elastic. If the human was strapped to a board and the board was anchored, the train would crush the human, but otherwise the human is going to be pushed out of the way, whereas something going much faster but with significant less mass is going to shatter the human because the elasticity of the human is not sufficient to disperse the force fast enough.