Tied beside a railway line (8)
I believe the answer is:
boundary
'line' is the definition.
(boundary is a kind of line)
'tied beside a railway' is the wordplay.
'tied' becomes 'bound' (similar in meaning).
'beside' says to put letters next to each other (I've seen this in other clues).
'railway' becomes 'ry' (abbreviation).
'bound'+'a'+'ry'='BOUNDARY'
(Other definitions for boundary that I've seen before include "line between states" , "Six, perhaps" , "Limit; hit at cricket" , "Limit, edge" , "Line determining the limits of an area" .)