Bath line's terminus in underground railway (4)

I believe the answer is:

tube

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But I can't explain it at all! Could you help me understand?

(Other definitions for tube that I've seen before include "Hollow cylindrical object" , "Pipe or hose" , "Long, hollow cylinder" , "Cylindrical hollow conduit" , "Pipe - railway" .)

I've seen this clue in The Sun.

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