I'm not sure there is a way to "physically join" separate RRT files, although SantaFe Jim's "select and paste" method might be worth a try. The end goal though is, you've got to learn to think in "layers" with RRT: keeping all the layers in ONE RRT file -- the same way photographers think in layers using Photoshop (if that helps).
Each of the track layers should have track in a different color. Scenery objects and buildings should be in their own layers too. When you're filling out the different layers with track components, you can "hide" other layers to avoid confusion. Then selectively toggle different layers on/off as needed.
Since all of the layers reside in one RRT file, you should start your RRT file with the maximum room dimensions... starting with the main track layer, then switch to the second (middle) layer and third (bottom) layer accordingly. I've also found it handy with RRT to plan ahead using absolute track heights. For example, in this plan, you might want to think about the main (top) track layer at 55", the second (middle) track layer at 48", and the third (bottom) track layer at 40". Or whatever works best for you.
Hope that helps.
David
