Bird's mode of transport (4)

I believe the answer is:

rail

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 "Bar; waterbird" , "wader" , "Complain bitterly - wading bird" , "A bird; bar to hang on" , "Form of transport" .)

I've seen this clue in The Guardian.

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