I have the same problem. I started a route 6 months ago , i only have track and textures down. I came back a week ago with a fresh install of trains 19 platinum and my parents ballast on my tracks has triangles throughout the whole route with pbr grass next to pbr ballast on track. If I change the pbr grass the triangles go away if I use any textures non pbr. I don't intend to use non pbr grass on my route. I hope there will be a fix for this. I am using a 5700xt I don't know if that's the issue, but I can't get a rtx video card in the u.s anywhere with a reasonable price. I suppose I will stop my route building until this can be addressed.
Windows 10 Pro,Intel 10600k at 5.1ghz, Rx 5700 XT
48 gb ddr4 gskill 3200mhz, 1 TB m.2 x2 raid 0 Trainz build 110491