Sure, the 15" cars might make it around the 31" curves, but how will they look doing it. I'd seriously consider the 13" O-27 Madison cars from MTH. They have full interiors, not the old-fashioned silhouettes, and look great. I have the O-27 Madisons from MTH in B&O, NYC, NKP, and PRR.
My new layout has larger curves with the outer mainline being 54" and 72" and the inner being 42" and 54", but I don't feel like these look too small. That said, I did just recently add a set of the MTH 15" Madisons. I cannot imagine them on my old layout which had 27" and 42" curves due to overhang.
