What is the minimum track radius for an N-Scale 2-6-6-2? I'm using the 15 degree Kato track and the engine seems to slow down, almost sticking in the curves.
You are mixing measurements in your question. The 15 degrees you mention would refer to the portion of arc covered by the curve (360 degrees being a full circle). That has nothing to do with the radius of the curve. Kato offers 15 degree curves in at least 6 different radii. Which one are you using? Your loco should handle at least an 11" minimum but will look (and operate) better on larger curves.
I had the same problem, its not the track radius. Look at the wires between the loco and tender. One or more of them is going to need repositioned because they are a bit stiff. You're going to have to play around a bit to find out which ones will need moved . But when they are all in the right spots it will go around the Kato curves without any problems......Mike