Class M-3/M-4 (Locobase 334)
Data from tables and diagrams in 1947 Locomotive Cyclopedia and from DM&IR 6 - 1951 Locomotive Diagrams supplied in May 2005 by Allen Stanley from his extensive collection. (Thanks to Chris Hohl for correcting valve gear ID.) Works numbers were 62526-62533 in May 1941; 64707-64711 in January 1943 and 64712-64716 in February.
These classic engines were based to a large extent on the Western Pacific's M-137-151 2-8-8-2 engines of 1931 (Locobase 332), but were lengthened to take an all-weather cab . The DM & IR engines burned bituminous coal and had a smaller grate area than the oil-burning M-137s although the overall firebox heating surface grew by 11 sq ft (1 sq m). In the M-4, this total included 194 sq ft (18 sq m) in three thermic syphons, 177 sq ft (16.45 sq m) in the combustion chamber, and 32 sq ft (3 sq m) of arch tubes. As part of the M-137 revision, the combustion chamber's length grew from 6 to 7 feet (2,134 mm).
Boiler tube and flue length (sheet to sheet) was shortened by 2 feet to 21 feet (6.4 m). The mixture of tubes and flues changed as the design substituted a Type E superheater for the M-137's Type A. The number of 2 1/4-in flues decreased to 82 and Baldwin installed 245 of the 3 3/4-in flues (vs. the M-137's 270 of the former and 75 5 1/2-in flues) . As a result, superheater area increased by 25%. The first five M-4s had Worthington feedwater heaters, the last five Elescos. Four piston valves each measured 12" (306 mm) in diameter.
Following contemporary practice, the M-3s had integrally cast frames and cylinders, Timken roller bearings on all driving axles, ASF (M-3) or SKF (M-4) roller bearings on all truck axles.
These engines performed very well from the time they entered service in May 1941, soon handling train loads 25% heavier than the earlier M-1/M-2 engines could manage. The last ten engines were M-4s, built by Baldwin in 1943, which used carbon steel in certain components because of a shortage of high-tech alloy steels. They had an engine weight of 699,700 lb (317,379 kg).
During 1943-44, as many as 12 "Yellowstones" were operating on the D & RGW, the GN, and the NP under lease where they were highly regarded. In fact, the D & RGW telegraphed the DM&IR with the claim that these Yellowstones were the finest engines ever to run on the Rio Grande.
The Lake Superior Railroad Museum site -- http://www.lsrm.org/Museum/mallet.htm (visited 9 Feb 2004) -- gives some interesting consumption numbers: " When working at full power, [the locomotive] could consume some 10 to 12 tons of coal an hour and evaporate water into steam at the astounding rate of 12,000 gallons per hour. The amount of coal ...used in one hour would be enough to heat a home for two winters." (and Minnesota winters at that, one supposes.)