Limitation of what is left, including railway transport (9)
I believe the answer is:
restraint
'limitation' is the definition.
(I know that restraint is a type of limitation)
'what is left including railway transport' is the wordplay.
'what is left' becomes 'rest' (I've seen this in another clue).
'including' is an insertion indicator.
'railway transport' becomes 'train' (I've seen this before).
'rest' placed around 'train' is 'RESTRAINT'.
'of' acts as a link.
(Other definitions for restraint that I've seen before include "Curb" , "It keeps someone or something under control" , "Moderation" , "Controlling measure" , "Self-discipline" .)