When the U.S. entered World War II The American railroads saw increases in traffic. The Baltimore and Ohio Railroad wanted to purchase more of the diesel locomotives since they were showing improved performance over steam locomotives. But the War Production Board had regulated the production of new steam and diesel locomotive designs until the war emergency was over. As a result along with producing 40 T-3 4-8-2 type locomotives, the B&O took delivery of 30 EM-1 "Yellowstones" built from 1944 thru 1945.
The EM-1 produced 115,000 pounds-force of tractive effort on 64-inch drivers with 235 pounds per square inch steam pressure and four 24-by-32-inch cylinders. Each engine produced ~5,900 Boiler Horsepower. The tender carried 22,000 gallons of water and 25 tons of coal. The engine weighed 627,000 pounds while the tender weighed 328,000 pounds for a combined 1,010,700 pounds. Nothing bigger could operate within the B&O main line due to the tunnel clearances and track restrictions that were in place.
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, IN until the B&O started to retire them in 1957. As good as they were, they could not compete with the diesels. None were saved from the scrapper's torch.