The polling thread about what curves sizes encouraged me to start this spin-off thread.
Even though Lionel's traditional 15 inch aluminum passenger cars were designed for O31 curves, they look awkward running on them, O42 curves are somewhat better, but there are other choices.
The traditional Heavyweight "Madison" passenger cars, at 14 inches in length look great on a O54 curve.
The "Baby Madison" cars, at 13 inches look great on O42
The O27 sized passenger cars, at 12 inches look just fine on O31
All measurements shown are body measurements
Interesting that Lionel made these cars in 1 inch length increments trying to meet everyone's needs I guess
Here is a pic of a set of 15 inch'ers rounding a curve on the WVRR that is a combo of O42 and O54. The lead-in to this curve is O72. IMHO, even these cars would look better on all O72.