Marias Pass 5 texture problem

Enjoying myself still after discovering Open Rails, I had another bash also at Trainz (TRS2009, for the first time since I got it, having started with the original product and later TRS 2004, many moons ago). This time, I starting off by getting 'Along UTA Lines' downloaded again, then fixed and working...





...and then moved on to some of the US lines featured in the 2009 edition, not an area of interest really but US route and locos look to be something that Trainz does quite well, like the Harlem Line route with its odd 3rd rail car body electro-diesels and shiny EMUs...





...and then the Marias Pass Approach route, part of the payware MP route by HP-Trainz...





By this convoluted route (sic) I got interested in running the MSTS Marias Pass, and thence to the improved MP5 version available over on Trainsim.com. This has improved terrain contours and scenery including farmed field textures and more lineside vegetation...



...but it also has some graphical glitches in Open Rails (stock or Monogame). Specifically, in a few places such as near Whitefish (pictured) and at Shelby, there are some unattractive sandy 'stretched' low-res textures - which tend to stray over the trackwork, as does a grass texture in the same areas (one of these just about visible about 100m ahead of the loco in the pic):



There doesn't seem to be a fix for this available on Trainsim.com, nor reference to it online, anywhere I can find.

I'm fairly sure I installed everything right - the installme.bat file didn't find any files until I stopped trying to 'Run as admin' and just double-clicked the thing at which point it copied over the stock files apparently just fine.

The only thing I didn't do per the Readme was check that I had the latest version of X Tracks installed. any pointers welcome!

Ivor