Car crossing the track? (7)

I believe the answer is:

sleeper

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'car' is the definition.
(sleeper is a kind of car)

I don't understand the rest of the clue.

(Other definitions for sleeper that I've seen before include "Railbed block; railway car" , "Dormant potential spy" , "Rail supporter; secret agent" , "night train" , "Possibly ring" .)

I've seen this clue in the Evening Standard.

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