Mallet Compound

Originally patented by Anatole Mallet in 1884, the articulated compound locomotive was developed to its greatest extent in North America, and at one period was the standard American heavy-freight type. Subsequent development, however, favoured the simple articulated, and it was left to the Norfolk & Western Railway to multiply its fleet of true Mallet compounds.

Since the ' Y2 ' class of 1918, successive enlargements culminated in the magnificent ' Y6 ' engines, eighty of which were built between 1936 and 1952. Weighing 495 tons, Nos. 2120-2199 were the most powerful conventional steam locomotives in the world; their two vast low-pressure cylinders ahead of the smokebox had a diameter of 39 in., the high-pressure ones of 25 in., and with a boiler pressure of 300 Ib. and 4 ft. 10 in. wheels they developed a tractive force of 170,000 Ib. Twenty earlier compounds of Class ' Y5 ' were rebuilt to Y6 standard, and these 100 locomotives were instrumental in keeping. The Norfolk & Western at a peak of operating efficiency matched by few other railroads in the U.S.A.

Long after the majority of lines were completely dieselised, the Norfolk & Western hauled 90 per cent of its traffic with three standard designs of steam locomotive, the ' Y6 ' class, the 4-8-4 type seen on page 110, and ahigh-speed simple articulated 2-6-6-4. As late as 1958 the ' Y6s ' were still running up to 6,000 miles per month hauling 13,000-ton coal trains from the mines of West Virginia down to the wharves at Lamberts Point; within months of this picture being taken diesel power had replaced steam and the ' Y6s ' had been retired