Quote Originally Posted by skibo View Post
I love those compound locomotives, this one looks to be a Challenger, with twelve drivers, it was smaller than the Union Pacific's Bigboy, but the Challenger had more traction effort than Bigboy's 16 drivers, due to less weight per driver!
According to this article:
https://www.railarchive.net/bigboys/index.html

Union Pacific Big Boy tractive force = 135,375 lbf
Union Pacific Challenger tractive force = 97,300 lbf

Usually you want more weight on drivers to increase traction (air foils on racing cars improved tractive performance when they were introduced).