Sign used on the railway in many books (5)
I believe the answer is:
libra
'sign' is the definition.
(sign of the zodiac)
'on the railway in many books' is the wordplay.
'on' is an insertion indicator (as in clothing 'on' a person).
'the railway' becomes 'br'.
'in many books' becomes 'lia' (I can't explain this - if you can you should give a lot more credence to this answer).
'br' put within 'lia' is 'LIBRA'.
'used' acts as a link.
Can you help me to learn more?
(Other definitions for libra that I've seen before include "Balance constellation" , "Old pound" , "House" , "Someone born between September 23 and October 22" , "Scales constellation" .)