Question

why is damage more when a stationary train is hit by a moving train?

Answer 1

when a moving train hits a stationary train the moving train travels with a lot of velocity and the stationary train has 0 velocity so conclusion the moving train has more power and thus it has a larger impact

Answer 2

The moving train has a lot of velocity and immense mometum so it will apply  a lot of impulse on the train at rest so it will have a lot of damage.If they were moving in the same direction the damage would be than compaared to the first case cause the resultant momentum will be less.
And in the case of the trains moving in the opposite direction there would be a tremendous amount of damage cause the momentum would be the sum of the momentum of both the trains.