Infact, some of the MDDs at NH are 'naughty boys' from the mainline: im pretty sure they went straight into the role without any kind of conversion course.
The last ex mainline 'naught boys' in 2011 took 4 months to pass out. What takes so long was the practical depot driving/shunting with a minder MDD, the traction conversion/fuelling only added a few days on that.
It takes a while to get used to shunting around the depot during busy periods, the communications side of it, and doing the odd moves like the half-units and swingers. It's not something anyone can just walk into, even ex-mainline. It's a bit different to following signals all the time.
Driving the train is the easy bit, the hard part is learning how to perform the role when its busy and there's lots happening at once.
Mainline are only short staffed because (I believe) Rest Day working is banned for Northern West?
If RDW ever came back in I still don't think they'd send Man Vic drivers up to NH to help out as it would require months of depot driver training with a minder. Being competent to drive to instruction and prep on NH, and being competant to cover MDD Shifts, are two completely different things. In my training, the former took me 4 weeks to learn, the latter took me over 5 months. Years later I've still not mastered it.
MDD'ing is seriously underestimated. I made that mistake myself. When I got the job, I thought it was going to be easy, as on paper it sounded like a watered-down Drivers role. Jesus Christ, I was wrong. I never knew that even the Depot Training and Pass Out was 5X more pressure than any part of the psychometrics could produce. The only easy bit was the first six weeks which were training academy led.
I am here to ensure that others do not make the same assumption.
It's also worth mentioning Fuelling/Tanking while we're on the subject - Fuelling takes a few days to learn and pass out on it, but you only REALLY learn how to fuel when you've done at least 16 weeks worth of it unsupervised!
To be able to Fuel 100 engines and Tank 60 Toilets in one night without getting covered in Diesel, and to be able to perform the role quickly & safely at the same time, is something that some people never master in 10+ years!