I started out as a guard mate, then pestered them like fook!
Be prepared for loads of sycometric(?) testing, and basic maths skills, engineering questions, communications and the ability to listen!
It's about 2 years start to finish of contstant assesments but once your there, they do tend to leave you alone abit with just the odd ride out and random download.
Good luck in your search!
Edit: have a look here
http://www.railwayregister.care4free...ain_driver.htm half way down is a section on making your application, and a general idea of who's taking on