The train driving training takes 12-18 months. First is 8 weeks of Rules (16 weeks if it's interspersed with time spent in the cab shadowing a driver). I imagine the principle of it is similar to ATPL theory, most of your time is in a classroom with a series of exams on things like signalling, emergency evacuations etc.
Next up is Traction. This time it's looking at the systems on the trains you'll be driving (isolation cocks, circuit breakers amongst many other things). Can last from 4 weeks to 3 months depending on what you have to learn and whether your employer has a centralised defect management. Think of it as a type rating without driving a train.
The longest bit is out with a Driver Instructor, you'll spend typically a year with one of these driving real trains with more than a thousand real passengers on them during the peak, I imagine it's broadly similar to line training in the aviation world. The Driver Instructor is a fully qualified driver who's most likely on over £50k a year. At the end of it all you'll have a 5-7 day exam with your manager covering everything you've learned. If you pass, you're now a driver.
Don't want to digress further, after all it's a thread about ageism in aviation, not trains. Nontheless, I do think the airlines could learn a few things from the railway world when it comes to recruiting older folk, who more often than not turn out to be very good drivers.
I hear that some of the niche sectors within aviation are a bit more open minded than some, plus some of the big boys will look at candidates who are 40-ish, though from what I can tell it gets difficult at 35-plus. Obviously, instructing is a different world but if I were in the left hand seat of a multi-crew aircraft I'd probably want someone with a bit of life experience next to me.