I've just been doing a bit of drivetrain testing with a view to standardising coding of drivewheels, bogies etc for any future work. We shouldn't have gauge wars on RT3. It's enough hassle already.
What I've found is that if the bottom of the drivewheel is set to zero height it will actually render as slightly below the track gfx. This gives the wheels a slightly chopped off appearance. If the wheel is coded to put the bottom of the outer rim at +0.1 units, it all looks good (this is CD CC CC 3D in the hex).
For the inner flange, having it set to -0.1units at the bottom looks right (this is CD CC CC BD in the hex). Obviously this also means setting it 0.2 units outside the outer rim at front, top and back. This gives a flange which looks realistic in width compared to everything else.
Using just 0.1 and 0.2 as the difference makes things easier for decimally-minded humans who want to chuck corrections into a base converter.
For width, setting the inner flange at either + or - 2.8125 units, depending on if it's on the right or left side (00 00 34 40 or 00 00 34 C0 in the hex) puts it just on the inside of the rail graphic, which is where it should be. This works out at 56 1/4" if using the 10" = 1 RT3 unit scale I worked out before, which is about right.
The outer flange has to go at 00 00 50 40 or 00 00 50 C0 to look right. This is 32 1/2", so 65" total. Looks like due to the pixel count in the track gfx it's approximating a top flange about 4 1/2" wide instead of about 3". That's wider than it really ought to be for standard rail profiles, but close enough for RT3.
Also figured I might as well do the wheels with four layers. This is what some of the default locos have (American 4-4-0 for example) and it tends to give a better effect at close range.
The wheels files usually only have one instance but I'm thinking they could easily have a few more if necessary. This would cut down on rendering power when zoomed out, and for simple things like wheels is easy to code manually. So it'd have four layers close up, and then drop down to one layer for the equivalent of the C or D skin.
The default wheels that have multiple layers don't bother doing this (they just stay multiple all the time) so I'll try them like that first. If we think we need to cut polys later on, adding the extra instances wont be much work.