In addition to the above, when HD Terrain first became available in Trainz Plus I did a trial conversion of my two largest routes, both built at 10m resolution, to the new HD resolution. Only one of the two routes had a single baseboard out of hundreds that was over the 16 texture limit, and that was without me ever counting how many textures I had used. But I appreciate that some creators do push the limits (I believe it is 256 textures but could I be wrong there) for non-HD terrain.
The main problem with converting old routes to HD is the increase in the size of the resulting file. In my case one route went from about 57MB in size to over 300MB. Others have reported that their large routes, after conversion, were far too big to be saved as a .cdp file (they exceeded the size limit) and thus could not be uploaded to the DLS.
A HD terrain 10m x 10m grid square contains about 16,000 more data points than the same grid square at 10m resolution.
There is at least 1 HD Terrain route on the DLS - <kuid2:642837:100268:1> Aston Heights - Switching industrial Layout and it uses up to 16 textures per baseboard without any obvious aesthetic defects.