you need a 'sponsor' - be that either Network Rail (the people paid by government to maintain the lines), or a contractor such as Amey, Vital, Jarvis etc. When you find someone to sponsor you, they will technically be your employer, and they will be able to send you on a course to obtain your PTS card (personal track safety card) without which, you arent even allowed near the tracks! You're better off working for Network Rail, as there will be more chance of progression, and a more reliable steady working environment. Once you have your PTS card, you will probably start doing basic duties, and as time goes on you will be able to gain more certification, such as Lookout, or Controller of Site Safety etc. You will have to decide between what department you want to work in, i.e. Permanent Way (tracks and crossings etc maintenance) or S&T (signals and telecomms maintenance). It's a good industry to be in, and I'm glad you're not listening to any of the nonsense the media spouts out about railway work. Good luck!