Sorry, I didn't notice there was more than a page 1 when I posted this response. The Challenger class was a 4-6-6-4 wheel arrangement. The BigBoy was 4-8-8-4. Allegheny was 2-6-6-6 and Yellowstone was 2-8-8-4. The BigBoy was not the largest locomotive ever built. All the locomotives listed above are articulated locos with two sets of drivers. There were a number of articulated locos with three sets of drivers known as Triplexes. They didn't perform well and not many were made. For more on steam locomotive class names see Steam Locomotive Classes. By the way, this all dovetails nicely with a discussion going on the MRR forum about a supposed "Southern Pacific 2-6-6-2 BigBoy" made by Rivarossi and to be marketed in the US by Hornby America. The SP guys are rather amused. Best
