Sumpter250 EOY 2010 challenge

[Sumpter250]

Have you considered using Model Aircraft Neoprene fuel line? This may easily solve your drive problem. You only need to cut the fuel line the proper length between the Worm gear's shaft and the motor's shaft. It will pull apart easy enough if you have to, but when the motor turns on, It clinches down on the shaft, kinda like a Chinese Finger trap (you know the one, where you can't pull your fingers apart?). If the locomotive ever binds, the fuel line will twist off rather than let your motor burn out.
Its actually a very reliable technique, and i've used it to repair a couple P1K RDCs and an old Metroliner (pictured below), as well as tried it in an old Athearn locomotive. Again, its probably cheaper and easier than to try and make a solid drive shaft. it is flexible and will stretch and twist with the tender.

I tried that with my HOn3 2-4-4-2, and it was great until the first curve. with a curve in the neoprene tubing, it was just stiff enough to cause problems with the tender.
Because this is a standard gauge loco the tubing idea might just work. Once I have the drive ready to test, I'll try both the tubing, and the cup-and-ball universals, and see how they compare.