If my minimum visible track diameter wasn't 104" on the mainline and 88" on an infrequently-used reversing loop, I would not use 21" cars. IMO, even 96" is tight for these cars. The 18s are OK down to O-72, but look better at 84 and up. A passenger station siding track has 96" curves, but I generally restrict that to a pair of RDC cars, which don't look as odd as a full train of 21" cars.
