Tressfx is mainly used for realistic hair. I could not find any reliable sources mentioning it used as grass.
I use PBR textures exclusively, mainly because they are sharper and better defined than standard textures. The only issue I have with them relates to using them on slopes (cuttings or trackbed) where the acid trip movement is most apparent. In an effort to combat the effect, I hide the slopes with either vegetation or a cliff spline where possible. Trackbed splines also have the advantage of being able to be placed high enough to negate the worst effects of PBR and allow TurfFX to be spread beneath the tracks which gives better coverage.
Take Elder Scrolls Online as an example, it uses PBR textures, TurfFX and Clutter as well, but the main difference is that in gameworld development Zenimax/Bethesda/Microsoft have almost unlimited resources and professional developers, whereas with Trainz route development we are virtually amateurs playing with professional tools. I'd love to be able to say "Art department, I need a rockface 40 metres high by 100 metres wide to hide some wonky textures", but with Trainz I just trawl the DLS hoping someone has made that rockface, because I have no idea how to make one.

Graeme you described very well the situation as is right now and funny how to cover up stuff not working but very true!