Auran programmed Trainz Carz to float above the ground at 0.40 when driving on a road, to match Auran's roads which float 0.45 above the ground.

So if you have a road that does not float 0.40 meters above the ground, the carz will be floating that much up in the air above the road, like hovercraft!

However, floating carz stink much less than floating roads!
If you really want to change the floating level for those roads, then if you are lucky, you should see a field named "grounded". This field tells Trainz how far to offset the spline mesh from the origin oin the Z axis. With Auran's built-in road, the mesh file has the road already floating 0.05 meters (?) above the origin, but the config.txt contains the field "grounded 0.4" (or something like that), which tells Trainz to offest the mesh 0.4 meters up on the Z axis.
That extra 0.05 meters above the ground in the Auran roads, regardless of whether the "grounded" field is specified or not, is actually a good idea. With flat roads, you really do want the road to be floating
at least a little bit, because if it is flush with the ground, the flickering effect will appear at a very short camera distance from the road in Trainz, and the road may be buried in the ground anyway if it is on a slope. Making the road float just a little will add a bit of camera distance for good viewing before the flickering starts.
So, go into Trainz and determine how much negative Z offset on the spline points you had to get in order to make the road
perfectly flush with the
flat ground. Then subtract 0.05 from that (or whatever), e.g. if you had to type in -0.45 to make it flush with the ground, record that as 0.40. Go into the config.txt file, and look for the "grounded" field. Subtract your recorded offset from the "grounded" field's value, e.g. 0.45 - 0.40 = 0.05. That should make a road that floats a negotiable amount above the ground (to make you have to zoom out further before the flickering starts), but yet looks much better.
If you do not find a "grounded" field in the config.txt file, then that means that the offset is specified in the mesh file, instead of the config.txt file. You can try adding a "grounded" field with a negative value in the config.txt to see if that does anything, but I do not know for certain if it will. In that case, take your recorded value, make it negative, and make that the value of the new "grounded" field, e.g. if your recorded value was 0.40, you would type "grounded -0.4".
I hope that this information is helpful.
Regards.