Lucius Beebe was also an author of a few other books and alot of his pictures showed steam at work in front of large trains. One of his pictures showed a 2-8-2 mikado lugging 15 or so cars up a 1 to 2% grade and then another would show the same type of engine running 70+ out of town on the flat.
I recall that a C&O 2-10-4 was designed and built to carry 160 empty hoppers between Cincinati and Toledo needing only a helper at the Ky/Ohio State line where a extremly large bridge with a serious hill awaits.
The B&O 2-10-2's that is on the internet with data from a demonstration run out west was able to handle trains at a fuel and water savings as well as do the same work somewhat faster than the equivilant power of the time.
The 2-8-8-2 y6B mallets were tops in the east coast mountains as they were able to keep even the biggest coal drag moving almost regardless of the terrain. One of Beebe's photos showed THREE of these at the head end and you can only imagine how many loads was being shoved and dragged over that line.
However the Norfolk and Western insisted on using 2-6-6-4's to get the same coal train rolling at 50 mph or more.
I think at Hinton they would use the Desiel Switcher to assist a steam powered train to break the inertia sorta like having two soldiers hoist a paratrooper with full kit heavier than he can carry into the airlift plane. Once moving the steam could keep it going.
The Cotton Belt 4-8-4 known as the Project 819 that is in Pine Bluff Arkansas undergoing restoration was said to be capable of hauling 18-25 passenger cars at some speed. Some of the videos shown on the engine indicated behavior of such power that it looked as if it was very difficult to keep the throttle closed until the entire thing was clear of the station and railfans.
The Cass Senic Railroad had shays. Some of which were the largest ever made. There is one part of the railroad where you can stand on one switch and lean over to look down one way and look up on the other track these engines were not very fast but you thought twice about walking down that hill.
The B&O railroad used Yellowstones 2-8-8-4 to get trains out of Brunswick Maryland (the farthest east they were permitted) past Harper's Ferry and westward up a punishing grade of some miles. I think they call this one the Sand Patch but not sure.. need to double check that. Anyways all west bound trains usually pound across Harper's bridges trying to gain as much momentum and speed as they can There are some hills beyond that into the region of Cumberland Md and up towards Ft. Necessity that rivals anything the west can offer.
I have heard recordings of the Cresent Limited, East Broad Top and other locomotives under steam and heavy loads laboring under grades in thier regions and you have to wonder if they would make it to the top before burning up all the fuel and boiling off all the water. The safetys would be crying all the way uphill.
And finally the PRR had engines such as the J1 2-10-4 built to handle wartime traffic at tonnage ratings very close to the mighty articulateds of the day. Other engines such as the 4-4-4-4 duplex had so much pull they could not find the traction to use that pull. It literally would get spun off and wasted until they could get the train moving. Once they got a hold..stand back!
I think they created a triplex and found that the boiler was too small to keep the cylinders fed at anything over 20 mph. And the ill-fated Jawn Henry could start anything man could hook up but only move at 4 mph or so they said.
Steam or Desiel the problem is the same... The bigger and heavier the train there is always a point as described in the earlier posts that it defeats itself.