Yeah, TLR is still in beta. There's a new iteration coming out soon(tm) for testing. Let's hope that version runs better.
That is one humungous route! As you add in content, you are going to run into file-size issues. The content isn't added to the route directly and is referenced by data-files. As you add content, the files grow in size as they track assets and textures.
That aside, you'll be landscaping that route for the rest of your life! I thought my own large route was big and I started mine 20 years ago, not kidding, and there's still parts I've been updating and rebuilding as well as "finishing off". Note the quotes... Like a regular model railroad, a Trainz route is never finished even if it appears that way.
TransDEM is awesome and addicting indeed. I've been working on a route for a friend of mine. He doesn't use Trainz but is interested in railroads. His grandfather and uncle worked for the Erie Railroad and initially for the New York and Greenwood Lake railroad that became an Erie line eventually. He lives in Greenwood Lake, NY and recognized the landscape after I showed him screenshots. He then pointed out places and named people that lived nearby.
The route was wiped out in the late 1920s and early 1930s when a second dam was built on the Wanaque River. This wiped out a large portion of the line and a good part of the valley. When he showed me the area, I went looking for topo-maps and got lucky. I found some from the mid-1930s that still showed the railroad even though the current DEM has the dam and water in place. Using some artistic sculpturing, I was able to dig out the river valley by using the topographic lines as a guide. I also rebuilt a hill near Pompano Lakes Jct. where Tilcon Inc. and the interstate had hacked the side of a hill. The old 1930s topo-map here still had the heights even though the hill looked like a snow cone with bite taken out. The biggest issue I have is the Wanaque Dam. This is the original reservoir dam and not the new one which I removed. This is quite unique with unusual brickwork and will require some modeling help when I get there. I'll artistic license the rest using actual photos and assets I can find.