The railway wouldn't bother me at all. Train noise is impersonal, unlike people noise, and at least you know there'll never be some hideous development built overlooking your garden. Railway land usually has an endemic rat population though so you'll need to be ready for that, scrupulous with your rubbish and proactive if any of your neighbours aren't.
I'm astonished that whole garden is yours, rather than shared or communal. Looking at the front of the building, it looks like the garden goes double or triple wide, as well as it being a 3-storey building, so basically you'd have sole use of a garden designed for 6-9 dwellings. I'd prepare to be unpopular! Have you actually bought it or just made an offer? I just ask as you don't seem completely familiar with the terms of the lease, and if not then use of the garden is absolutely something I'd be getting my solicitor to nail down, as it's a big part of the flat's value imo. It's very nice, though, and South Norwood is okay really - safe, just not necessarily very snazzy.