Amtrak vehicles (6)
I believe the answer is:
trains
'amtrak vehicles' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for trains that I've seen before include "Public transport vehicles with carriages" , "Works out" , "Exercises" , "follows exercise regime" , "Pieces of cloth trailing from the backs of gowns" .)