Trainee train drivers vacancies are rarely advertised. And when they arise TOCs usually tend to look for qualified drivers from other companies or do internal recruitment first. So to maximise your chance as an external applicant you have to devise a planner.
Make an effort to check TOCs website every week on the career section. Some TOCs may not have vacancies but will let you fill in an online application form. This if you do will make it easier for you should a job come up. All you will then do is to tweak it little to suit the essential requirements and submit. Always emphasise your relevant experience and skills. This is your initial selling point. Doing this will increase your chance of being selected for assessment day.
Another plan is to tailor you CV to suit the trainee train driver job – the skills and experience outlined in the advert. This will make life easier when filing in any future application forms.
Phone or e-mail them. Whichever is better for you or both. There have been circumstances where some external applicants have been allowed to come for assessment although the vacancy was never advertised externally. (The who you know factor)
You can apply for another role first, for instance a Conductor or Customer Service Host. With this route you get to hear of vacancies and apply for them before external applicants are given the opportunity.