Crazy railway engine (4)
I believe the answer is:
loco
I believe this clue is a double definition.
'crazy' is the first definition.
(loco can informally mean mad)
'railway engine' is the second definition.
(I've seen this before)
(Other definitions for loco that I've seen before include "Engine (abbrev.)" , "with a screw loose?" , "Transport" , "Engine - nuts" , "Potty" .)