Tower Hill reached by rail shortly? (7)

I believe the answer is:

tractor

I'm a little stuck... Click here to teach me more about this clue!

'tower' is the definition.
(I've seen this before)

I don't understand the remainder of the clue.

(Other definitions for tractor that I've seen before include "Farming vehicle" , "Agricultural machine" , "Agricultural vehicle" , "Bombardier possibly" , "Powerful farm vehicle for heavy work" .)

I've seen this clue in The Sun.

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