Glen is correct. Actually, I don't believe that it was until the 1990's that diesels began to achieve the horsepower & tractive effort of steam locomotives...but they could be MU'd to reduce crew costs, better performance at low speeds/on grades...and maint costs. F units were 1500 horsepower...hence a medium sized locomotive...such as a 2-8-4...was comparable to a 3-unit lash up in pulling power. The 1 f unit definitely could not pull a 15 car passenger train...but 1 would be fine for a 2-3 car train. 1 F-unit could pull around 10 passenger cars...so you'd want to have a second on for reliability. If you had a twenty car train...you'd want 3 F-units minimum for if you lost 1. 4500HP is ample to pull a 34 car passenger train at 85mph.