HD Terrain is a big change in approach from the old grid system. With 1,600 times more data points than the old 5m grid, we utilize a number of different techniques to allow far greater detail while still ensuring similar performance.
To achieve the flexibility and quality we're looking for, there are necessary trade-offs between performance, visuals, and accuracy. One of the main differences is a new limit of 16 unique textures per baseboard (rather than the original limit of 256 textures per route).
If you attempt to paint a 17th texture, the least used texture on that baseboard will automatically be replaced by surrounding textures.