Conductor's place (3)
(Other definitions for pit that I've seen before include "Hellhole?" , "where orchestra plays" , "Deep hole (in theatre?)" , "Coalmine" , "Quarry" .)
'conductor's place' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for train that I've seen before include "Rail service" , "Prepare for public transport?" , "union's lengthy item" , "Do exercises" , "become physically fit" .)