I’m planning on leaving my 12 year career in the RAF....
Plenty of ex-RAF, and other armed forces, police (constables and higher up) firefighters, paramedics, and prison officers. There's also insurance salesmen, McDonalds workers, a radio DJ, a carpet fitter, a supermarket manager, someone who worked in a scientific laboratory in Cambridge.... lots of people who now have a new career as a train driver.
I wouldn't worry too much about only being in a job for a short time - you'd have 12 years in the RAF previously, which would show you'd been stable in your employment. They'd look at the whole picture, and if you got an interview you'd be asked why you want to be a train driver and you'd be able to explain why.
On point one, do you have some suggestions of companies i should look out for? And is there a term used to describe a a hub where train drivers start work? Like a hub or depot? Like I don’t expect a shift to begin at my local station, would they all start at large stations like Kings Cross? Sorry if I’m coming across quite naively
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DB Cargo (a freight company) have 'hubs' which is a location where you are normally based but can be sent out to drive in your car or company van upto 60 minutes away before starting work.
All the TOCs (train operating companies - passenger trains) have you book on for work at the 'traincrew depot' you work at, and finish there at the end of your working day. If you have to pick up your train from another location, you will travel to it as a passenger during your working time.
With GTR, Hitchin depot was downsized and work moved to a new traincrew depot at Welwyn Garden City. There is a waiting list of drivers to move back to Hitchin, so you won't have any luck at the moment for Hitchin I'm afraid.
There is a waiting list for Welwyn Garden City as well, but some of the people on it haven't been productive for two years so can't move depots yet, so there may be recruitment that leapfrogs these.
Hornsey depot is more likely to have vacancies when there are any. The earliest start is something like 03:45, so you might have to drive for that duties like that one.
Sometimes though, drivers book on at Hornsey and travel to Finsbury Park to pick up a train. If you had a duty like that, you could arrange to book on at Welwyn traincrew depot and travel straight to Finsbury Park which would save you time.
GTR also have depots on another route (so perhaps more difficult to get to) at St Albans and Cricklewood.
Arriva Trains London (London Overground) have a traincrew depot at Chingford - don't know if that's too far for you?
Greater Anglia has a traincrew depot at Bishops Stortford as mentioned above - and also the point was made that advertised vacancies often close early.