Room for top railway (6)
I believe the answer is:
vestry
'room' is the definition.
(I know that vestry is a type of room)
'top railway' is the wordplay.
'top' becomes 'vest' (I've seen this before).
'railway' becomes 'ry' (abbreviation for railway).
'vest'+'ry'='VESTRY'
'for' acts as a link.
(Other definitions for vestry that I've seen before include "Storage and dressing room in a church" , "meeting of parishioners" , "Church room where clergy dress themselves" , "Small room attached to church" , "Parochial meeting-room in church" .)