Old-fashioned railway system runs for miles (5)
I believe the answer is:
retro
'old-fashioned' is the definition.
(I've seen this before)
'railway system runs for miles' is the wordplay.
I cannot really understand how this works, but
'railway' could be 'r' (eg in Great Western Railway) and 'r' is located in the answer.
'runs' could be 'r' (cricket abbreviation) and 'r' is located in the answer.
'for' could be 'to' (eg both can mean 'in the opinion of') and 'to' is present in the leftover letters.
A single letter 'e' remains which might be clued in a way I don't see.
This may be the basis of clue (or it may be nonsense).
(Other definitions for retro that I've seen before include "Prefix for old-style" , "in nostalgic style" , "Backward looking" , "Reminiscent of a past style" , "imitating past fashion" .)