Mark's mock-up clearly shows the problems encountered with long (and high) cars on very sharp curves. Another thing to take into consideration is clearances for tunnels and bridges (both through and under.)
Having spent a fair amount of my early model railroading in HO, I tend to think in HO terms. R20 is slightly less than 14" radius in HO. Nobody in HO makes (or even worries about making) 80+ foot cars in HO for that radius.
The defacto-standard radius for HO is 18" and seems to be drifting up to 22" as a minimum requirement for long cars. R27 is roughly the equivalent of HO's 18" radius. Back in the day I was running full length passenger cars and 86' freight cars on 18" radius, but there still would occasionally be derailments due to the forces on the curves.
Here's an American Models 85' passenger car on R27. I set this curve up as both a test curve and interchange track on my railroad:
As can be seen, the overhang is still quite severe, but almost manageable. An 86' auto rack or high cube would have slightly more outside overhand due to the location of the trucks. Truck mounted couplers would mitigate problems when coupled to shorter equipment. My mainline curve is 33" radius and these full length cars still look a little awkward and occasionally cause problems.
AM's 72' heavyweights and non-scale length Superliners and Budd cars look and work much better on my "generous" curves.
Rusty
