HD Terrain is different than HD screen resolution. HD Terrain allows for a higher amount of height points per baseboard than 5m grid allows. Currently HD Terrain is in alpha testing with a select group of testers and they are rebuilding the HD Terrain based upon the feedback given. I imagine it will go through several rounds of alpha testing before going to beta testing which is regular Trainz Plus version. Trainz Plus will be able to use, create and edit HD Terrain routes. Once Trainz 22 is updated to support HD Terrain, Trainz 22 can run HD Terrain routes. At least that is my understanding.