Thank you so much for all the help, everyone!
ray_whiley: Had a look at that page, some great stuff! I think I'll have a flick through model railway plans, both 3D and 2D track layouts and try to incorporate them into my routes in areas where I'm struggling.
leksie: You really can't beat a bit of Thomas the Tank Engine

I think I may look at some stills of older episodes where you can see a large portion of the track and get a feel of how to incorporate plans into my layouts. This would work very well with branchlines, as a lot of the track in Thomas is single track
JCitroen: I've never thought to create a history for the route. I have an idea at present for a mainline connecting two major towns, with two branchlines coming off it at various points and a line coming off one of the branchlines to a coal mine, using British DMU's and loco's, so I bet I could engineer a history for it!
JIb228: Field trips are a great way of getting everything right. Where I live in Manchester, we have a rather extensive Light-Rail system and I'm determined on one-day recreating it so field trips would be easy for that

it's getting the inspiration to create made-up routes that's my problem at the moment, but track plans may help with that
cascaderailroad: Well, a 2.5% grade is about 1:40, so taking your lowest and highest point of -3000m and +3000m, that would take 6000:240000 so roughly 240 kilometres or about 150 miles

Never thought of doing submarines! I guess the underground coal mine route would be easy with some tunnel splines and digholes
Vern: I always tend to build things in the wrong order. I make a stretch of track and then signal it almost immediately...probably not the best idea, eh? I think if I talk on TrainzChat with people, I'll keep the wheels rolling (aha)
trainman7616: The Marias Pass takes me back! I remember that on Microsoft Train Simulator! I often find myself picking up a piece of paper and just doing a quick 2D sketch of a route. More often than not, though, it goes in the bin after 5 minutes
Thanks for all the help, everyone! You've given me some great tips!
Regards, Jack