DerCal,
The guys here have given you some really great advice so far. Building a route, whether large or small can be a big task which should take a very long time. Why did I say should? Well it's relatively easy just to lay track and plop buildings and stuff down much like we'd do for a loop around the living room or under the Christmas tree. But... to get a route that is worth driving takes a lot of work and lots and lots of time.
Now having said this, there are other considerations too. You've thought and mentioned your brief thoughts on the route. One of the biggest things that helps is what is known as the back story. This means having a history of the railroad's operation, whether present or previous, and using this to keep the plan consistent and as a goal seeker.
So you've come up with a route that's modern and runs in the summer. Where is it located?, What kind of trains run? Is it a short line, a branch of a major operation, or perhaps a tourist line? Perhaps a bit of it all... You see you're thinking at the top level and too broad. This is a first step in defining your route, but you need to take this and narrow down things a bit. It's a great idea to investigate, Wikipedia, your local library, even a good internet search of a railroad you are interested in will give you ideas for your route, combine this and with a bit of imagination, you can now come up with a good song and dance - a story of your railroad. Let's use a small short line as an example...
The Shoreham and East Sandwich was built as a branch off of the Eastern Railroad in 1878. This line served the fishing industry, located on the east side of Sandwich Bay, as well as some passenger service between East Sandwich, Little's Cove, and Shoreham. The service was successful and by 1888 this line, like the rest of the Eastern Railroad, was absorbed into the ever growing Boston and Maine Railroad, which by this time was now operating everything in northern Massachusetts, New Hampshire, Vermont, and parts of Maine. The passenger service remained popular up until the late 1950s when it was discontinued in favor of bus and car service as the B&M gave up short-turn passenger service and focused on freight. (This is true by the way). By the early 1970s, the freight service was dwindling as well as the fishing industry closed and freight moved to trucks. By the time Guilford Transportation Industries (GTI, now Pan Am Railways) took over, the line was mothballed by the state, (pick, Maine, New Hampshire, Mass.) for a potential future use or even a rail trail. In the mid-1980s some local business men saw the potential for a tourist operation and rebuilt the line for summer operations... You can fill in more.
So now we have an old seacoast branch line that had some freight operation before, but now has only a tourist train running. What kind of equipment? This is now another question that needs filling... How about a 44-tonner pulling some old B&M or New Haven American Flyer coaches. Perhaps throw in a dining car for a dinner cruise service... We're getting adventurous here! The thing is let the imagination grow, but keep things in perspective and within the story that you created.
So now we start building. You can use existing Trainz DEM files which are up on the DLS, or generate your own with TransDEM. At this point, though, being a first route perhaps, let's keep it simple and we'll make our own landscape using the existing Trainz tools. We squish, and lift, flatten and manipulate a little bit of topology at a time. We lay some track, and now we're on our way to building a route. Being eastern New England, we can't have steep mountains coming down to the shore so you'll need to flatten things down. I recommend looking at Bing Maps, bird's eye view, and even Google Earth and going into Street View there. Look at stations, trackside views, streets, downtowns, etc. to get an idea what the area looks like.
I mentioned sticking to the story. The reasons are many, but you'll find that it really does help in keeping the railroad together. What you don't want to do, although no one is stopping you from doing it, is have an oilfield next to a big canyon with skyscrapers across the street. This will look odd and also it doesn't fit the theme. Another reason is it makes building the route easier by narrowing down our selection of assets we want for your route. On the DLS, plus installed, there are close over 300,000 assets. This doesn't count any that you can obtain from other websites, or even payware. And yet another reason... Should you decide to give up the route and start it again later, all this ground work is already done and you can rebuild this again and again without having to remain stuck in this step one rut.
So anyway, no matter how big or small a route is, you need a backstory and defined period as this helps bring the rest together.
John