Bird's mode of transport (4)
I believe the answer is:
Here is my best explanation:
I believe this is a double definition.
'bird's' is the first definition.
(rail is a kind of bird)
'mode of transport' is the second definition.
(I've seen this before)
(Other definitions for rail that I've seen before include "protest", "Once it was British", "Fence", "Safety bar; bird", "length", "Complain about - marsh bird", "Mode of transport; bar", "Use abusive language", "Bird with long legs" and "Bar of rolled steel".)