I remembered 0.3 m from way way back but when I searched last year I also found nothing definite that specifies top of track has to be at z = 0.3m in the track spline mesh. The old chunky mesh track definition was what I ended up finding and it has a diagram that shows it as 0.3 meters. It was 1 of 7 values you had to input with the chunky-info tag. That's almost 20 years old and I'd bet the farm no one is modeling this kind of track anymore. With the new procedural track using individual rail assets you'd think it would be something that needs to be specified - for new content makers.
The wiki "how to guide" for building track is mia.
Bob Pearson