(For greater speeds, the Union Pacific Railroad chose a four-wheel leading truck and drivers of 68 inches (1.73 m) for its Big Boy 4-8-8-4 class.)

The Southern Pacific Railroad's famous "cab forward" articulated steam locomotives were effectively a Yellowstone in reverse (4-8-8-2).

This was done to spare the crew from the heavy smoke output of the large engines on the former Central Pacific, where tunnels and snow sheds were common and lengthy.

The need for a larger, coal-burning firebox and a longer, all-weather cab led to the use of a four-wheel trailing truck, giving them the "Yellowstone" wheel arrangement.

They were the most powerful Yellowstones built, producing 140,000 lbf (620 kN) of tractive effort, and had the most weight on drivers so that they were less prone to slipping.

The second batch was completed in late 1943 after the Missabe's seasonal downturn in ore traffic, so some of the new M-4s were leased to and delivered directly to the Denver and Rio Grande Western Railroad.

The next winter, the D&RGW again leased the DM&IR's Yellowstones as helpers over Tennessee Pass, Colorado, and for other freight duties.

[1][page needed] This was despite the Rio Grande's earlier assessment that these Yellowstones were the finest engines ever to operate there.

DM&IRs were the only Yellowstones to have a high-capacity pedestal or centipede tender, and had roller bearings on all axles.

Some of the locomotives had a cylindrical Elesco feedwater heater ahead of the smoke stack, while others had a Worthington unit with its rectangular box in the same location.

Clare shops in Baltimore, Maryland, the B&O took delivery of 30 class EM-1 Yellowstones in 1944 and 1945, the largest number and the smallest of this type built by Baldwin as well as the most modern.

The EM-1 produced 115,000 pounds-force (510 kN) of tractive effort on 64-inch (1.6 m) drivers with 235 pounds per square inch (1.62 MPa) steam pressure and four 24-by-32-inch (0.61 by 0.81 m) cylinders.

They were equipped with the newest technology, including the Worthington feedwater heater, superheater with front-end throttle, Cyclone front end, thermic syphons, a lateral cushioning device in the front pair of drivers on both engines and the front wheels and the trailing truck, as well as roller bearings on all axles, engine and tender, which gave them the reputation of "yard creepers", because three men could move one on a level track with the cylinder cocks open.

Fleet numbers 7600–7619 were built and delivered in 1944 and 7620–7629 in 1945, all by the Baldwin Locomotive Works of Philadelphia, Pennsylvania.

Since the EM-1s had roller bearings throughout, they also handled mail and express trains, replacing two B&O class T-3 4-8-2 Mountains.

[2][page needed] The EM-1s also ran on the Pittsburgh Division over Sand Patch Grade near Meyersdale, Pennsylvania, with empty hoppers, iron ore or dolomite loads westbound, coal eastbound, as well as general freight in both directions.

Well-known photographer and Cumberland, MD native William P. Price captured, on still pictures and 8mm films, the EM-1s on the east side of Sand Patch pulling heavy trains with two of the B&O's 2-10-2 class S1 and S1a Big Sixes on the rear as helpers dispatched from Hyndman, PA. Near the end of steam they were all sent out to Fairmont and Wheeling, West Virginia, and Lorain, Ohio, with lake-bound coal trains as well as runs between Willard, OH and Garrett, Indiana, until the B&O started to retire them in 1957, and all of them were scrapped.