Getting ready for the tube coming down the line? (7,2)

I believe the answer is:

rolling up

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'getting' is the definition.
The answer and definition can be both related to change as well as being present participle verbs.
Perhaps you can see an association between them that I don't see?

I cannot understand how the rest of the clue works.

Can you help me to learn more?

I've seen this clue in the Sydney Morning Herald.

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