Mmmm, so I have to understand that actually Terrain HD doesn't work outside of the Plus version.
Correct. You can load a HD route created in Trainz Plus into TRS22PE but it will be "dumbed-down" to the normal Trainz (non-plus) resolution. TRS22 (standard) will not load a HD route at all.
As pointed out by
@wreeder above, the SD and HD options in the Route Editor window only apply to HD routes in Trainz Plus - why they also appear in TRS22 is a mystery to me as well.
In Trainz Plus selecting the HD option in the Route Editor gives you access to all HD features and controls including a brush size for terrain and textures down to 0.12m (12cm) in size. Selecting the SD option also gives you access to all HD features and controls but with a minimum brush size of 2m - unlike a 2m brush under 5m/10m resolution routes the brush will not be restricted to the grid lines (it will not turn red in areas where it will not work). The SD option gives you much faster terrain and texture painting in HD routes in areas where the finer detail given by smaller brushes is not needed.
In my opinion HD terrain is a complete waste!
I have to disagree. There are currently some very good HD routes but all are small in terms of the number of baseboards. That, to me at least, is the first hurdle that needs to be overcome. As
@PhilChorusch01 points out in post #5 above, HD is a huge resource eater - a HD baseboard contains 6400x the data of a 10m baseboard and 1600x the data of a 5m baseboard so a "medium" sized route in HD can easily exceed the size limit for uploading to the DLS (about 500MB) or even for saving as a .cpd file (maximum 1GB). The compression level used with Trainz Plus and TRS22PE routes was recently increased significantly but still not enough to deal with large routes. More work is needed there.
Phil also points out other issues in HD that will need to be looked at. But you have to start somewhere. How often have users complained in these forums about the lack of higher detail (or even "video quality") graphics in Trainz for the terrain and textures? HD is, in my opinion at least, a first step in that direction.
I don't use HD terrain simply because my current project is far too big - I did do a trial conversion to HD which took many minutes to complete but the resulting file size in CM was eye-watering. Plus it did not (and obviously could not) automatically convert low resolution terrain into high detail - that I would have to manually do myself.