Thanks for the detailed post. Could you please describe further for me a "floating track and road?" After some searching it appears these are tracks and roads that "float" above the terrain? Why does that happen?
This is an issue mainly with older types of tracks, but the problem still exists in some cases due to how the alpha channel is implemented on these assets. This causes a space under the tracks. In other cases, the ballast on the older assets is nothing more than a strip of printed paper, well not really, but that's what it looks like! This causes the tracks to move across the landscape like a piece of paper tape placed down on the surface. There are places where the ground doesn't quite fit underneath because the resolution of the ground is higher than that with the track. As I said, this is and was an issue with the older assets.
With roads it's a combination of things. First there's that alpha-channel issue where like the tracks if you are to level the terrain underneath, it pulls the ground away from the spline! The more you smooth and tamp, the more the ground goes away. People resorted to sinking the roads by about -0.2 meters, which was the offset between the open space and the surface.
To add insult to injury Auran, then the original developer of the Trainz program, oh so many years ago now going on 20 years actually, did some incorrect things and set the roads too high at 3 to 4 meters off the surface. To add insult to injury, the developers then corrected the car traffic so that the cars would be set to ride on the surface of the roads rather than in them. Then Auran at the time, in another update fixed the roads by lowering them, but forgot about the cars. This caused floating vehicles, which you will never see now, such as the famous built-in A-Team white van that always showed up at rail crossings just as you were about to take a screenshot.
Then finally in TRS2009/TS2010, Madeline Usher, aka Maddy25 came up with her YARN roads. These set the standard for roads that had some thickness to them. This fixed the floating road issue, more or less, and compensated for the floating cars (Carz). These road series was expanded on by Streadway and others as they created variants of the same series with the Yarnish roads. This wasn't without it's issues either during this transition time, there were times when the new Carz, now a different class than the original vehicle class, and not a track object (more about this), where they got their wheels buried again as people fixed the height of the original assets. Yes, it got a bit complicated. With TS12 HF4 (Build 61388) and up including T:ANE and TRS19, vehicles are now track objects, and roads are no longer classified as splines separate from tracks. They are in fact a kind track. With this now, the vehicles no longer float.
But getting back to the track and roads floating. With some newer assets, this is still an issue which route developers seriously need to look at and watch for. There will be places where you can't help it due to how the splines bend versus the tighter bends of the terrain, but those really are the exception, and the diligent use of retaining walls, wall splines and other assets can fill in this space nicely. The problem is some route builders don't seem to pay attention to that, and that always bothered me even in the beginning. Today it's worse with the shadows because it's more obvious that there is a space underneath.