The Decapod 2-10-0 of the Spectrum kind is a relatively small engine. The drivers on the prototype were in the range of 62-65" diameter. Even though it had 10 drivers, it was capable of negotiating normal curves, and the Bachmann edition does the same...so 18" is just dandy. Each additional inch in radius, regardless of the engine in question, is going to reduce the probability of binding and derailments related to less than optimal curvatures that much more, and it is probably inversely exponential.
At one time I felt that a Decapod couldn't possibly take tight HO curves, but a few kind people quickly disabused me of that misunderstanding.
-Crandell