Since you don't want to use actual DEM height data, you can simply place images of the landscape you want to model. There are two different ways to do this, which don't involve using TransDEM, though TransDEM can probably do this as well, but at the cost of some extra work as well to generate the individual UTM tiles.
This in the end is the traditional method using images that placed on individual image-assets. Each tile is 1024 x 1024 and your image is scaled accordingly. You then place these just below the surface and then build your route on top of them.
This process used to be done manually, and the problem, however, is each image needs to be chopped up out of the larger image using your favorite image editing program, and then added to each and every base map asset manually, which really creates a lot of extra prep work before building the route. Then once placed on a route, they have to be locked in place on a separate layer each one individually.
With the introduction of Trainz Model Railroad 2017, user ModelerMJ introduced a utility called BaseMapz, which generates the individual tiles from an image. This will scale the image to fit a specific area and then trim the overall image into multiple images that are placed on BaseMapz-assets automatically. These then automatically export and a route is created with these assets in place all ready for building on. The other advantage too is all the images are placed on a single layer automatically which can be deleted later once the route building is completed.
Read this here as it has more details and includes a link to the program, which is included free with TMR2017, but is available for download by all Trainz users at the author's website.
http://online.ts2009.com/mediaWiki/index.php/Basemapz