To add to christherobot's post I too work on the railway and found that my company at least didn't discriminate when it came to age and external candidates.
When I attended the psychometric testing day, of 10 candidates, 6 were internal and 4 were external. Only two of us made it to the end and we were both external candidates. There was a trainee driver on my course who was 52 and had never seen the inside of a train cab before in his life.
I've now been on the railway for nearly 12 years and I can honestly say it's the best job I've ever had, I obtained my PPL in 2008 and considered the modular route to a career in aviation but never did in the end and I'm now happy to fly for pleasure.
During my PPL training one of the other students was a Driving Instructor for (then) Virgin Trains West Coast. He was happy to explain the processes of recruitment and gave me some handy pointers when applying.
As mentioned there are normally many more applicants than positions and the testing requires some preparation and practice.
Like Chris says above I'd rather be flying planes than driving trains but there aren't many jobs that pay 50k-60k basic for a 4 day week, strong union, every night in my own bed and overtime opportunities that see drivers at my depot earn over 100k.
I didn't have to pay a penny towards my training however I was bonded to my company for 3 years once qualified.
Just my tuppence worth.
Tommy