The EM1 class comprised thirty locomotives (numbered 7600-7629) built for the B&O by Baldwin in 1944 and 1945.  These articulated engines maintained 235 psi steam pressure, and had 24×32" cylinders (4) and 64" disk drivers.  They exerted 115,000 pounds of tractive effort, and weighed 1,010,700 pounds (including tender) in working order.  The 12-wheel tenders held 25 tons of coal and 22,000 gallons of water.  EM1s had roller bearings on all axles, a feature which made them so free-rolling that, with the cylinder cocks open and on level track, one of these behemoths could be pushed by three men.

The EM1 was among the smallest of the Yellowstone (2-8-8-4) type ever built, but nothing larger would have conformed to the Baltimore and Ohio's tight clearances.  Except for some minor staybolt problems, stemming from the uneven distribution of water in the long boiler on steep grades, the EM1 was an exceptionally fine running and reliable locomotive.

The last "new" steam power built for the B&O, EM1s were initially assigned to the mountainous Cumberland and Pittsburgh divisions.  Later, as diesel-electric power displaced steam in the Alleghenies, they were shifted west to the ore-hauling region in northern Ohio.  Though designed primarily for heavy freight service, they were versatile machines, and in the early years were even used as passenger power on occasion.  As a group, the EM1s lasted until the end of steam power on the B&O.  Their ranks began to thin in 1958, however, when the decision was made to cease all major expenditures for steam locomotive repairs.  Overhauls in progress were halted, and EM1s due for major maintenance were slated for scrapping instead.  By 1960, all of these magnificent giants had gone to the torch; not a single one was spared.