I have never noticed any temperature rise when running my modern O gauge locomotives. Who needs to run for eight hours continuously? And I doubt there would be any temperature effect even then. In particular, I have two MTH Premier PRR G-5s locos (PS-1 and PS-2) that have been running trouble-free for more than twenty years. I don't think that design could be considered inadequate. As for the 4-6-6T - we shall see.
MELGAR