Sets of railway cars (6)
I believe the answer is:
trains
'sets of railway cars' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for trains that I've seen before include "follows exercise regime" , "Pieces of cloth trailing from the backs of gowns" , "Prepares by practice for performance" , "They're drawn by locomotive" , "Prepares for competition" .)