I've been looking at this, in particular with UK Bullhead, the lods are too vicious and IMO far too many and not all bits are changing at the same distance.
Worth remembering that andi06's protrack which seems to have been the base for a lot of the non PBR protrack can be modified and bits used as per his license, not quite sure why he used m.reflect on the sleepers though which have a shiny glass appearance in TRS19, possibly a slip up as using the same texture for track and sleepers, fixable by dumping the m.reflect.
Main reason I decided to investigate was I have a industrial area which in prototype has extremely sharp curves, track I am using has a 3m mesh that as soon as you back off view wise, switches to about 10 m and you get a series of straight lines joined together however the sleepers (ties) remain in the original position, so depending on the view the track is flipping from a curve to these horrible straight bits. Then of course there is the random disapearance of bits of the turnouts (switches)
Interestingly the 2ft DMT Protrack I'm using for the Narrow Gauge part does not have that problem bits stay put and track doesn't vanish when viewed from a distance.
I'm not convinced that PBR is really necessary for track either, unless you sit looking at ballast from a couple feet away all the time, you are not going to see the detail and just basic normal mapping would be fine.
I'm also wondering if having large mesh libraries for track covering different variations, is having a negative effect on performance and stability of pro track.