The question, “Why do you want to be a shunter?” should be quite easy to answer. It’s the other bit that usually follows which tends to throw people, “Why do you want to work for us in particular?” Or words to that effect. In reality and as has been said here, most people aren’t bothered about the TOC specifically and apply to whoever is recruiting and whoever is a commutable distance.
For the first question, I’d say something like, “I want to progress within the railway industry and want a role that will challenge me, and which requires great responsibility”. For the other question about why them, I’d usually say something like, “I want to change to another TOC as I’ve been with my current one for a number of years. It would be easy for me to apply for my existing TOC, but I want to work for another respected company within the industry”.
Admittedly that’s only possible as I work for a TOC already, but even if you don’t, saying something like, “I want to work in the railway, and I know you are respected by your employees/people and look after your staff and customers” blah blah, all the corporate stuff.
I also wouldn’t mention salary, which I think is common sense (only at the end of the interview if it hasn’t been explained). I personally don’t actually want to be a driver for the salary anyway, although it’s obviously a bonus! I know salary is often a big selling point for jobs on the railway, so I can see why people might be swayed by it (not just driving jobs, but a fair few jobs in general).