What's the difference between Railworks Challenger DLC, and The UP Big Boy DLC?
They are both articulated locomotives; the big difference is that each power section on Big Boy has four coupled axles (4-8-8-4), while Challenger has three coupled axles in each section (4-6-6-4).
Both have two-axle leading trucks and a two-axle truck under the cab.
Big Boy develops 135,375 pounds of tractive effort versus 97,400 pounds for Challenger.
Big Boy is heavier at 540 tons vs 486, and longer—40.487 meters vs 30.16 meters.
Neither is “The most powerful, heaviest, etc.†The DM&IR M3 Yellowstone and the Chessie Allegheny are arguably entitled to that title (for US engines) depending upon what criteria you choose.
The most powerful steam engines during the last 25 years of the Age of Steam were the Beyer-Garratt 4-8-2+2-8-4 locomotives that hauled freight from Mombasa up to Nairobi on East African Railways.