A shunter doesn't do any driving, a shunt driver does it. A shunter guides trains throughout the depot and fuels them amongst other duties. I've only ever seen shunt driving jobs advertised internally but these are now virtually non existent due the amount of ex mainline drivers who now do this, for reasons such as SPAD's, overshoots and the like. It's very much like a disciplinary tool, though a lot of them seem to enjoy it. Anyway, it's best to keep your eyes peeled if it's something you fancy. There will be jobs of all sorts listed around april or may on the website I think. I'd advise you to join in any capacity, then you can apply for internal jobs, a lot of which don't get put out externally.
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What I posted above applies to corkerhill depot. I know the shunters at shields depot are different as they move the units also. Not sure about haymarket, eastfield or other depots.