Re: Which could take a tighter curve?

Kevin,
If that question realy interest you, I suggest you invest in the book from the NMRA called "Porter Steam Locomites" which is a reprint of their 13th catalogue that gives the tracking values for every size engine.
Small 4-4-0 could run on 150' radius.[sharpest advised]
Smallest 4-6-0 used 150' but sharpest practical was 100' radius.
It all depend on the design of the specific engine and some times a front driver could have been blind so it would really track anything.
It is even know that some engines were built with blind outer drivers and only the inner pair of a 8 driver engine had the flanges. All kind of designs were made. Some designs did not work well.
The book also gives much of value for track requirements on narrow gauge with the data used in the days when miles were being built.