Conservatory adds colour to railway (8)
I believe the answer is:
orangery
'conservatory' is the definition.
(thesaurus)
'colour to railway' is the wordplay.
'colour' becomes 'orange' (orange is a kind of colour).
'to railway' becomes 'ry' (abbreviation for railway).
'orange'+'ry'='ORANGERY'
'adds' is the link.
This may not be correct. It may be part of another bit of the clue.
Can you help me to learn more?
(Other definitions for orangery that I've seen before include "cultivated area" , "Place for growing fruit in cool climates" , "fruit source" , "greenhouse, maybe" , "where fruit trees grow" .)