Bath line's terminus in subterranean railway (4)
I believe the answer is:
tube
'subterranean railway' is the definition.
'tube' can be an answer for 'railway' (tube is a kind of railway). I'm not certain of the 'subterranean' bit.
'bath line's terminus' is the wordplay.
'bath' becomes 'tub' (synonyms).
'line's terminus' becomes 'e' (I am not sure about this - if you are sure you should believe this answer much more).
'tub'+'e'='TUBE'
'in' is the link.
Can you help me to learn more?
(Other definitions for tube that I've seen before include "TV" , "Pipe - railway" , "Can of beer (colloquial)" , "train network" , "Hollow cylindrical object" .)