Question

Why does a person sitting in one train think that the other train is at rest, when both the trains are moving on parallel tracks, with the same speed and in the same direction.

Answer 1

This is because the relative velocity of the train in which the person is sitting with respect to the other train is zero.

For example,

The velocity of 'A' train = 50 km/hr. due East and the velocity of 'B' train = 50 km/hr. due East.

Relative velocity of A train w.r.t., B train = 50 - 50 = 0.

And similarly Relative velocity of B w.r.t., A = 0.