[tzs]

Is momentum actually what matters in collisions?

Consider a 9 000 pound car at 50 mph. If you are standing there and that hits you you are going to die or be seriously injured.

Compare to a large freight train hitting you at 1 mph. A bit of Googling suggests that the largest freight train had a mass of around 140 000 000 pounds.

The momentum of that freight train would be more than 300 tims the momentum of the car (and the kinetic energy would be 6 times that of the car). Yet a free standing person hit be that train probably won't be killed or even seriously injured by the collision.