The Big Boy is a hair under 700 tons and produces 6500-7500 hp depending on the weather and how good the engineer is. many books and documents say the 4000's produce 7000 hp, others say it was 8000 hp and most are true to  say its actual 6300 hp but 4000's were known to have fluctuating hp like all steam locomotives and that they could still produce alot more hp than there calculated hp. The 4000's could pull a fully loaded 5 mile freight train on level track. doing a quick calculation in my head they could pull roughly 20,000 tons! the 4000's pulled nominal 1 mile 4500 ton trains up it's famed Sherman hill and if you do the math and multiply the 4,500 times the 5 mile train it could pull on level track you would get 20,000 tons!! The 4000's were very sure-footed machines and rarely did one see it spin it's drivers while lugging it's 100 cars up Sherman hill. The 4000's were able to sprint at 80 mph but delivered there best hp at 37 mph where generally any where from 6500 hp and 7500 hp was developed. They could cruise at 60 mph on flat track with a heavy train but this speed was hard on the tracks and was prohibited to only 35 mph after shopping and when new. You would think that the almost 22 ft long 9 ft firebox would be hard to fire but actually I heard in Trains Magiziene from Steve Lee that they were actually some of the easiest engines to fire that was working on the road.