Whilst I am not a train driver, I am intimately familiar with the railway industry from a professional point of view and have as part of that worked with both many train drivers and also (replacement) bus drivers, the latter presumably coming under the PCV banner AFAIK. Train driving is a world apart from road vehicle driving / riding of any sort, and apart from concentration and reaction times, you would probably find little in the way of similarity between the theory of PCV and train driving, let alone the practical aspects. Train drivers for mainline companies will usually undergo circa 12 months of training, including Rule Book theory, emergency procedures, company-specific issues, traction training (technical training for types, ie. classes, of train) and route learning (memorising and being tested on driving according to every line feature, including stations, gradients, signal positions and hazards, signal box information, junction/tunnel/bridge/landmark names - you name it really!). Then, after a fair bit of experience (usually at the least a few years with a very good safety record) you would perhaps tentatively look to become an instructor. It's not possible to take proper shortcuts in any decent (read: now that ORR/NR have got to grips with it, all) mainline establishments these days unless you already have some sort of train driving qualification and just need to certify on traction or routes.
Also, as above, competency and especially route knowledge must be refreshed and/or tested every few months unless there are very good reasons not to (eg. long-term sickness) - and in the latter case, you might need to partially re-qualify on some aspects.
A more flexible option might be to look to become a member of one of the leading heritage railways, then volunteer and build up skills as train crew, though becoming a driver would still take a fair while - and you'd almost certainly be limited to heritage railway tracks and slower speeds, without full mainline competency.