Fast train say (7)
I believe the answer is:
express
This clue is a double definition.
'fast train' is the first definition.
(I've seen this before)
'say' is the second definition.
(thesaurus)
(Other definitions for express that I've seen before include "At high speed like some post" , "Specific; going at speed" , "Convey in words; train" , "Intimate - squeeze out" , "set forth" .)