Tied to a railway line (8)
I believe the answer is:
boundary
'line' is the definition.
(boundary is a kind of line)
'tied to a railway' is the wordplay.
'tied' becomes 'bound' ('bind' can be a synonym of 'tie').
'railway' becomes 'ry' (abbreviation for railway).
'bound'+'a'+'ry'='BOUNDARY'
(Other definitions for boundary that I've seen before include "Limit; hit at cricket" , "4 possibly" , "Sharp division" , "Frontier" , "Border (especially in cricket)" .)