Question

How to find speed of train when train length and time are given?

Answer 1

Let x be the length of the train which includes the engine also. When the end of the train passes the object, then the engine of the train has to move a distance equal to the train.

Then the time taken by the train to pass the stationary object = length of the train/speed of the train

Answer 2

By relative velocity concept this concept  is very useful in mechanics  problem solving .