Crossword Genius

Train that's shunted from inactivity (7)

Ross

I believe the answer is:

inertia

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

'inactivity' is the definition.
(I know that inactivity can be written as inertia)

'train that's shunted from' is the wordplay.
I cannot quite understand how this works, but
'from' could be 'in' (I've seen this in other clues) and 'in' is present in the answer.
This explanation may well be incorrect...

Can you help me to learn more?

(Other definitions for inertia that I've seen before include "Apathetic state" , "Tendency not to act" , "Motion (if any) when no external force is applied" , "Lethargy" , "Resistance to change" .)

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