Guard posted on railway (6)
I believe the answer is:
sentry
'guard' is the definition.
(sentry is a kind of guard)
'posted on railway' is the wordplay.
'posted' becomes 'sent' ('send' can be a synonym of 'post').
'on' says to put letters next to each other (in a down clue, letters appear on others).
'railway' becomes 'ry' (abbreviation).
'sent'+'ry'='SENTRY'
(Other definitions for sentry that I've seen before include "One keeps watch" , "Lookout soldier" , "One keeps guard" , "Guard on watch" , "Look-out, guard" .)