Bars train supporters (5)
I believe the answer is:
I believe this clue is a double definition.
'bars' is the first definition.
(rail is a kind of bar)
'train supporters' is the second definition.
(I've seen this in another clue)
(Other definitions for rails that I've seen before include "Train tracks", "Barriers of horizontal bars and supports", "secretive birds", "Metal bars of a barrier", "Tracks or barriers".)