A good resource for layout plans for use in TMR is the classic Kalmbach publications, and in particular, the books '101 Trackplans for Model Railroaders' and 'Small Railroads You Can Build' (a plan from the later book I admit I used as a basis for a Trainz route I made a few years ago as a means of taking a break from my main route building, and it turned out pretty good). I'm not sure if online versions of these books exist, but you could try looking on Kalmbach Publishing's website and see if they offer them electronically (I bought my copies years ago from a model train store so they've been part of my collection for a long time).
Either way, they have some really good trackplans that would make interesting Trainz routes, either in TRS or TMR.
And speaking of Kalmbach books and Trainz, a Trainz rendition of the layout from the classic Kalmbach book 'HO Railroad that Grows' is on the DLS and well worth a look at too.
I have that Kalmbach book! I got mine back in the 1970s for about $1.75 at the local hobby store with money I earned cutting my neighbor's grass. I used to trace the big routes and dream about building them. There were some that caught my eye back then and I've been considering building them at some point in Trainz.
The plans need a bit of adjusting when brought into Trainz due artistic license to make the plans pretty for publication, but they are really nice plans that we can use. Imagine what Lin Wescott, John Armstrong, and the others would say seeing their drawings come to life in a virtual world!