According to my copy of "The Railroad - What it Is, What it Does"
by John Armstrong, you need roughly 2 hp/ton to go 30 mph on
a 1% grade, more than 1 hp/ton to go 20 mph, and less than
1 hp/ton to go 10 mph, on the same grade.
But you'll start pulling knuckles if all of your power is on the head
end and your train weighs more than about 10,500 tons on a 1%
grade - so you either limit to 10,500 tons and provide two or three
4,000+ hp units, or you use DPUs to push-pull a heavier train.
Coal trains can be 18,000 tons or more in some territories.
UP and BNSF often pull PRB coal trains with one or two 4,400 hp
units while pushing with one more similar remote unit. BNSF
runs some coal trains across Iowa with only one unit at each end.
I'm not sure what the ruling grades are on these routes.
- Ed Kyle