Although the Baltimore & Ohio wanted to buy diesel locomotives for freight traffic in the forties, they had to order steam locomotives due to the war restrictions. This was the birth of the class EM-1 with the wheel arrangement 2-8-8-4, called “Yellowstone”, being the last articulated of the B&O.

They were thoroughly modern locomotives with roller bearings on all axles including the tender. Thanks to five thermic syphons, arch tubes and a long combustion chamber, the firebox had an enormous surface of 756 square feet, larger than that of the Big Boy. With 117.5 square feet, the grate was smaller than that of the Big Boy that had to burn low-quality coal. Both lead drivers and the lead axle of the trailing bogie had some sideways play.

At the beginning, they operated in the Cumberland area with challenging grades. Services included everything from express freights to heavy coal trains, with occasional passenger trains. For the heaviest trains, they operated in double with another locomotive pushing at the rear. Later they came into the Pittsburgh area and were scrapped between 1957 and 1960.