Try to think of it like this. When you first walk through the door for your interview they are going to immediately start asking themselves some questions. Is this person honest, responsible, trustworthy? Will they be able and willing to learn everything they are taught and apply it in real world conditions? Would I trust them with a 5 million pound train and the lives of 600 passengers.
Write your cover letter with this in mind, starting from a position of confidence in your mind that you will make a naturally excellent train driver and are willing to put in the work which will get you there. It's very easy to be a lazy train driver and this is what managers hate the most. You have to have confidence but never be over confident. Lastly, try to articulate that you understand that it is a job which can very easily catch you out, you have to treat it with respect and make the right lifestyle choices to go along with it, early bedtimes before early starts etc.
Good luck!