I'm just bumping this because I think I may have run into the same (or similar) issue.
Last night I swapped out the GTX1060 card in my Trainz PC (running TRS19) for a new GTX1660. It's kind of the same card but with more modern 'stuff' in it, originally bought for use in another machine but things got in the way and this was better.
I upped the shadow and water qualities to the maximum (they'd previously been one down) and the only other change I made was to toggle 'Texture Streaming' from On to Off, as my understanding is that you don't need it with reasonably good cards.
Then I loaded my layout and lo-and-behold there were areas where the track was sunk into the ground. The track is (from memory) the TRS19 1t Wood and it wasn't happening everywhere, but there were several areas where it was very noticeable.
Now, I can't be sure right now as to what textures were in use but I'm fairly sure they're NOT PBR ones (although the comment above about the TRS19 tracks using PBR ballast is interesting) - however, I'd need to check. Is it safe to assume that all PBR ones will have PBR in the name? I'm guessing not.
I'm not complaining, I'm just curious as to what made Trainz suddenly decide to start eating sections of track. Could the Texture Streaming change be it? It's no biggie - in fact it'll force me to separate my textures into known PBR ones and known non-PBR ones, something I've been intending to do. I've been noodling around with this layout for ages under TRS19 with no issues of this nature. the only thing that changed was the graphics card, the shadow quality, the water quality and the texture streaming. The nVidia driver did NOT get updated (although it looks like there's an update available, so that'll happen tonight).
From the top of my head the textures I use the most are 'JT Grass 50' (my default 'just fill in the grid with this for now' texture), the Pinehill 1-6 grass textures, 1000m Ground, and a couple of Protrack ballast ones (along with a couple other ballast ones I can't remember, but I'll find out if anyone cares).
Any thoughts?