Once you have the height it's not that much more difficult.
Also, learn to hop onto a skate park rail by going fast and jumping early, so that there's a bit of a gap before you land on the rail. This will get you used to jumping on rails that go down stair sets where you need to clear a bit of a gap before you land on the rail.
I recommend learning as much as you can on the smaller rails before moving on to something bigger. I know the low rails get pretty boring but they're a good way to learn bike control and get you used to the feel of various grinds. That way when you move on to higher stuff you'll have that memory built up and it will make the overall progression easier.
It's a trade off though, depending on how low a rail we're talking about.
I used to practice on a 12" rail and I found that I learned a lot more a lot faster when I switched to a 1.5 foot or 2 foot rail. I think it's because I could actually throw myself into the taller rail, which made it easier to get more aggressive than I did on the 12" rail.