I want to help set the record straight re the B&O EM-1.
B&O had the most challenging mainline route of ANY US railroad. The maximum grade (Cranberry) is 2.2% uncompensated and is 2.87% compensated for curvature, after the additional resistance to traverse several 10 degree adjoining curves is considered. In comparison, the PRR old line on the Pittsburgh Div. was 2.2%, Norfolk & Western Roanoke to Bluefield was 1.34%. ATSF had some pretty severe grades did Great Northern, but none approached what B&O had to contend with. GN was electrified……
For mountain work, a B&O EM-1 had 64” drivers. To keep axle loadings as light as practical, an EM-1 ran at 235 psi, which would permit thinner boiler steel and resultant lower weight while maintaining the required ICC boiler safety ratio. This somewhat reduced max boiler pressure resulted in a calculated starting tractive force of 115,000 lb. Adhesive weight was 485,000 lb. resulting in a low factor of adhesion of 23.7%. The only other mallet design that MIGHT comply with B&O clearances and that could match the EM-1 performance on the B&O on Cranberry was probably an N&W Y-6. However, the Y-6 had an adhesive weight of 66,500 lb per driving axle, which may have precluded a loco of this axle loading in this service on the B&O. (The EM-1 had an individual axle loading of 60,625 lb.) With higher drivers, a B&O EM-1 would have been faster on the more level portions of the B&O than a compound mallet with 57" drivers.
A horsepower rating for an EM-1 has never been published, but it is possible to roughly determine what the horsepower might be from the trains that these engines hauled and their speeds in doing so. In the Staufer book “B&O Power”, an EM-1 unassisted on a QD (Quick Dispatch) train was rated at 1300 tons on Cranberry. If the EM-1 reached 20 mph with this train of 50 ton box cars, its drawbar HP would be 4230. If it could run at 25 mph with this train on this grade, its DBHP would be 5310. (I do not know what the uphill speed limit was on Cranberry uphill during the steam age, but this speed seems reasonable in view of current operations by CSX.)
The EM-1 was not a “large” Yellowstone type, but it was definitely not “small”! Of all the coal burning simple articulated locomotives built by Baldwin, the EM-1 was probably only eclipsed by the DM&IR Yellowstones and the SP AC class, which were significantly larger.
The calculated (using dynamometer car) drawbar pull and corresponding speed, and therefore drawbar HP, for a NP Z-5 Yellowstone was 5180 after being rebuilt after WWII with roller bearings, grate partially blocked, etc. This is from the Frey NP book. While the Z-5 is larger dimensionally and weighs more, it was a 1930 design, while the EM-1 was a 1940's design. The EM-1, along with the PRR T-1 and the ATSF 2900 Northern and 5011 Class 2-10-4, are probably the best steamers ever outshopped by Baldwin. My opinion.....