Watchman posted by railway (6)
I believe the answer is:
sentry
'watchman' is the definition.
(sentry is a kind of watchman)
'posted by railway' is the wordplay.
'posted' becomes 'sent' ('send' can be a synonym of 'post').
'by' is a charade indicator (letters next to each other).
'railway' becomes 'ry' (abbreviation for railway).
'sent'+'ry'='SENTRY'
(Other definitions for sentry that I've seen before include "One posted on guard" , "Military guard" , "Military lookout" , "Picket" , "Sentinel, watch" .)