In the FWIW dept:
Most house cars with horizontal brake wheels were built before ~1930, so I thought it might be helpful to review the relevant Safety Appliances standards that prevailed at that time. They were pretty bare bones:
1. wheel dia at least 15", preferably 16"
2. staff centerline 17-22" from car-end CL
3. at least 4" clearance from any surface around the wheel itself.
4. brake safely operable while car in motion
5. brake step optional
That's it. I can't find any minimum or maximum (or typical) height above roof or running board. It looks like a car-by-car decision.
I did not look at flat cars, gons, or hoppers.
Swall Meadows, CA