Matt,
For your first route, start small and build up from there. 6 years later and my first route is now over 180 miles long, end-to-end. LOL!
I use a combination of terrain first, then tracks, and then in some areas tracks first, then terrain. It depends upon the circumstances. In some areas, I've just laid out the track, such as in terminals, for example then went back and did the rest. In other cases, such as the rolling countryside, I'll work on the landscape first, then add the track in later. This in some ways is more fun because you have to plan the route around the least amount of grades.
If you import blank baseboard routes, such as DEM maps by Fishlipsatwork, you'll have essentially a blank terrain with hills already done. All you do then is to add your track and scenery. There's a lot of trimming that has to be done though in order to make them useable because they cover a lot of area.
I've used a few of these when building my route. They give a whole new element to the program. The thing is don't get overwhelmed by the amount of landmass in the route. Use what you need and trim (see above).
With the DEM landscape, you can follow the TIGER lines and lay your track on them, using the guides for a realistic track, and you can put in your own branchlines as well by surveying the ground for the best route. I did this with an area of my route. The Sandy Point branch of my route is an add-on to a DEM of the Bangor to Bucksport, Maine route that I imported into mine.
Here are some hints that I hope you find helpfule:
When making hills and valleys, get your eye level down to the track level and look around so you can picture what you're doing, and how realistic the scene looks. CTRL+Y in Surveyor I think will do this. Hit ESC to exit.
What I find helpful is looking at maps and what things are like in the real world, and then trying to imitate what I see. In some cases though, it's difficult to copy exactly what I see, but I can get the basic feeling.
Also, I forgot to mention, stick with a theme - a plan so to speak. Is this a regional railroad? A shortline, perhaps that connects to a major carrier. Try to use rolling stock that matches that specific theme. This may mean older locomotives and freight cars. You don't necessarily have to resking them for your own roadname if you don't want to. Many shortlines use turned in equipment that they just run with the old names still showing. Others just paint over the name, but keep the rest of the paint job the same.
The landscape too should be realistic. In other words no airport right next to the city downtown, etc. Again look and see what's done in the real life.
Vary your textures, and watch for repeated patterns. Use the [] keys to swirl the textures around and mix them up as you lay them down.
When you lay track, ensure that it is actually on the ground and not floating. This may mean getting the terrain height then putting that number in the spline height of the track. You may also have to fiddle about with the ground a little here and there.
These are some things to think about. Check out the Tips and Tricks sticky in the forum here for more information.
John