Cash that goes by boat or train? (5)

I believe the answer is:

gravy

'cash' is the definition.
(gravy can informally mean money)

I don't understand the remainder of the clue.

(Other definitions for gravy that I've seen before include "Meat juices after cooking" , "Meat jus" , "Sauce made with meat juices" , "Kind of train?" , "something saucy" .)

I've seen this clue in The Guardian.

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