Train's track (4)
I believe the answer is:
rail
'train's track' is the definition.
'rail' can be an answer for 'track' (synonyms). I also know that 'train' relates to this answer.
This is all the clue.
(Other definitions for rail that I've seen before include "Fulminate - wading bird" , "Mode of transport; bar" , "Scoff" , "Complain strongly or part of fence" , "method of transport" .)