In chains on a railway line (8)
I believe the answer is:
boundary
'line' is the definition.
(boundary is a kind of line)
'in chains on a railway' is the wordplay.
'in chains' becomes 'bound' ('bound' can be similar in meaning to 'in chains').
'on' means one lot of letters go next to another.
'railway' becomes 'ry' (abbreviation for railway).
'bound'+'a'+'ry'='BOUNDARY'
(Other definitions for boundary that I've seen before include "Line determining the limits of an area" , "march" , "Limit; hit at cricket" , "Sharp division" , "line between states" .)