Why during a head on collision between a moving train and a car travelling at the same speed, not much happens to the train but the car becomes totally crushed?

The force of impact is equal to both vehicles, so that means F \Delta t is the same for both. Consequently m\Delta v is also the same. But since the train's mass is so much larger than the car's mass, the train's \Delta v is significantly less than the car's. That's why when a train hits a car the car gets pushed backwards down the track, and not the other way around. For occupants on board the car the \Delta v may be lethal, whereas for the occupants of the train all they feel is a small bump.

However, your question seems to be more about the damage caused to the two vehicles. This is actually a function of how the train is constructed versus the car. Keep in mind that the impact force F is the same for both, so if the two vehicles were constructed of the same materials they would each suffer equivalent damage. But they're not. It's the much stronger construction of the train engine compared to the car that accounts for the difference in damage.