Going on using public transport (2,5)

I believe the answer is:

in train

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'using public transport' is the definition.
(I've seen this before)

I don't understand how the rest of the clue works.

(Other definitions for in train that I've seen before include "Moving forward" , "On the move" , "making progress" , "Being on board" , "Under way" .)

I've seen this clue in the Evening Standard.

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