Still in post on a railway (10)
I believe the answer is:
stationary
'still' is the definition.
(both can mean immobile)
'post on a railway' is the wordplay.
'post' becomes 'station' (a station is a post).
'on' is a charade indicator (letters next to each other).
'railway' becomes 'ry' (abbreviation for railway).
'station'+'a'+'ry'='STATIONARY'
'in' is the link.
(Other definitions for stationary that I've seen before include "Motionless" , "Fixed" , "Not moving" , "Unmoving" , "Parking" .)