I take it you are a Guard at the moment (judging by your username) so you could say that you want to progress in your railway career. You could say that your current role has given you a good insight into what a driver does and you believe that you would enjoy the role of a mainline driver and it would provide more job satisfaction.
You could also say that after being a Guard for X amount of years you are now looking for a new and challenging role and would enjoy learning new skills. You can also add that you would like to do more to help Northern build on its PPM to achieve its 94% reliability target and would enjoy helping passengers get from A to B (the sort of corporate stuff they like to hear I think).
You can also add things like you want the role because you believe you have the right skills to do it, and you could even say that you enjoy working alone as thats what you'll be doing if you get the job obviously.
I think mentioning the salary is not required as it goes without saying that the salary is a motivator, it would be like stating the obvious, so cant see that gaining any points in your interview score.