Siamese for one using iron railway (6)
I believe the answer is:
feline
'siamese for one' is the definition.
(I know that Siamese cat is a type of house cat)
'iron railway' is the wordplay.
'iron' becomes 'fe' (Fe is the chemical symbol for iron).
'railway' becomes 'line' (line is a kind of railway).
'fe'+'line'='FELINE'
'using' is the link.
(Other definitions for feline that I've seen before include "Like a leopard, say" , "suitable for Skimbleshanks?" , "Of or like cats" , "Cat-like" , "Persian for one" .)