With maths, further maths and physics, you'd be able to do physics, maths, engineering, astrophysics. and probably some other stuff too. If you do chemistry too, you'll be able to do stuff like chemistry, chemical engineering, natural sciences (you might be able to do this with out chem, probably depends on the uni).
If you want to work on the railway, I'd think probably physics or engineering would be best, but check an job requirements for stuff you're interested in.
And remember, it's much better to have 3 good/great grades than 4 mediocre/bad ones.