My advice to myself, going back in time, with the caveat that I never had a job in the railway industry, would echo
@bouff34 - make sure whatever you do is something you'll be happy studying for three years.
I started out at school with Mathematics, Physics and Chemistry but I switched to drop Chemistry and study Further Mathematics instead, and I was really good at them all, 3xA at A-level (no A* stuff back then), and I kind of fell into a Mathematics degree. Bad mistake, because it's a very different subject at university than it was at school, so I failed my exams, changed university, started a new Computing Science degree and had three years of this. More benign times because it didn't cost me or my family anything like what it now would cost - I'm talking 1980-84.
Wisdom of hindsight, I should have studied Physics. It's a subject that still interests me today, whereas Mathematics in my mind is just a tool, calculus is obvious, but Group Theory and Lebesgue Integration left me cold then and still do.
Whichever degree you end up with shows your ability to stay the course and learn I think.
I did have job offer from BR Nottingham back in the day, but I went to work for IBM in Portsmouth instead. In part because the day of the BR interview I had a job offer from IBM and a full pack of information on terms and conditions, which I read on the train to Nottingham.
So research the university course, which I didn't do, I just assumed because I'd been good at Mathematics at school I'd be good at university too.
Finally, that’s my advice to myself back in time, it may be interesting, it may be helpful, but it may also be totally irrelevant! Good luck, whatever course you pursue.