They may go up the steps and under the train (5)
I believe the answer is:
'they may go up the steps' is the definition.
I cannot understand how the remainder of the clue works.
Can you help me to learn more?
(Other definitions for rails that I've seen before include "Wading birds - dysfunctional person has come off them" , "Barriers of horizontal bars and supports" , "Tracks or barriers" , "secretive birds" , "Metal bars of a barrier" .)