As Oknosten mentions is the basemap objects. You simply take the map image, saved as a tga, and place it on the already created flat object. the image has to be 1024 x 1024. Activerail's method is similar, however, directly out of SketchUp using the RubyTMIX plug-in to create the flat objects. Which method you choose is up to you. In the end these objects get deleted from the route because they are only used to place track and objects. The pain with these objects is they can get in the way and also be hidden by terrain mesh. I will assume you are using TS12 and if you are you have the advantage of using layers to lock these objects into place. Since these assets are placed just a bit under the terrain, you'll need to use the wireframe view to see them. Once your objects are in place you can then change to the solid terrain and later delete the object as it's no longer needed..
John