Pulling power depends on many factors, weight on the drivers, length of driver wheel base, and is the weight well distributed on the drivers (factor of adhesion) and not on the leading or trailing trucks, gear ratio (the higher the ratio, the more pulling power), and the size and torgue of your motor. It is better if traction tires and magne-traction is not needed. Also a larger radius track is helpful. On the articulated steam engines, the usual drive system works well for the rear drivers, since most of the weight is concentrated there, while the front drivers have very little weight, besides what is in the frames, on them. The better pulling engines have two motors independently attached to each set of drivers and the weight can be used on both front and rear drivers. The Koh's articulateds, as well as those drive systems built by Stu Kleinschmidt, Sofua et al. fall in this catagory. Because of the relatively long wheelbase of diesels, even Geeps, a GP-9 can outpull a Big Boy, if the diesel has alot of weight on it's drivers giving it a high factor of adhesion, and the Big Boy has a standard drive system (where most of the weight is on the rear drivers). Because of their long rigid wheelbase, 2-10-4's and duplexes can take off with a long train when they are well weighted and have a good can motor. The best combination is one of the old Lobaughs (articulateds) with a Kleinschmidt/Sofua 2-motored drive. This is an unbeatable combination. Also, the old doorstops-Parmalee-Sturgis electrics, Alexander diesels and electrics are a good bet.