Specifically American type of railway in New York (6)
I believe the answer is:
namely
'specifically' is the definition.
(I've seen this before)
'american type of railway in new york' is the wordplay.
'american' becomes 'am' (abbreviation in some contexts).
'type of railway' becomes 'el'.
'in' means one lot of letters goes inside another.
'new york' becomes 'ny'.
'am'+'el'='amel'
'amel' placed into 'ny' is 'NAMELY'.
(Other definitions for namely that I've seen before include "Videlicet" , "to wit" , "Viz" , "That is to say, specifically" , "Old especially" .)