Mallet locomotive
The American engineer W. S. Hudson patented a system of compounding for railway locomotives in 1873[1] in which he proposed an intermediate receiver surrounded by hot gas from the fire, so that the low-pressure steam is partly superheated.
In 1884, he proposed compounding combined with articulation; on lightly engineered secondary lines this could give greater power to locomotives whose axle load and size were limited.
Typically the support bearing was placed beneath the smokebox, hollowed and with a sliding seal to provide a route for exhaust steam from the low-pressure cylinders to discharge through a blastpipe within the smoke box.
Large numbers of Mallet designs for narrow-gauge railways were built, but in 1889 the first six standard gauge examples were built by J A Maffei for the Swiss Central railways, and an 87 t (96 short tons) 0-6-6-0T banker (US: pusher) for the Gotthard Bahn, the last being the most powerful and heaviest locomotive in the world at the time.
[5] Received negatively at first due to speed limitation arising from the short wheelbase and stiff suspension, it gained support during service, and it was soon followed by Baldwin examples, and then steadily heavier and more powerful successors.
Their in-house compound 0-6-6-0 design located both the high and low pressure cylinders adjacent to one another in the center of the locomotive driving opposite directions.
The units were unpopular with crews owing to frequent steam leakages and derailments resulting from the lack of pilot wheels.
The largest compound Mallets were ten 2-10-10-2s built for the Virginian by Alco in 1918; in pairs they pushed coal trains headed by a 2-8-8-2.
[8] The AT&SF also had a number of compound 2-10-10-2s, assembled in their own shops from existing 2-10-2s using a kit, supplied by Baldwin, consisting of the front 10-wheel frame and a boiler extension.
Although compounds had been considered obsolescent since the 1920s, C&O thought them appropriate, in the late 1940s, for low-speed coal-mine pickup runs converging on the classification yard at Russell, Kentucky.
The last compound Mallets to remain in use on a major North American railroad were the N&W class Y6b 2-8-8-2 locomotives, retired in July 1959.
By about 1920, the U.S. version of the Mallet as a huge slow-speed pusher had reached a plateau; the size of the low-pressure cylinders became a limiting factor even on the large loading gauge permitted in the U.S., and reciprocating masses posed serious dynamic problems above walking pace.
Moreover, there were adhesion stability problems where the front engine tended to slip and then stall uncontrollably because of an imbalance of tractive effort and axle load, accentuated by the drawbar reaction, and inability of the intermediate steam receiver to accommodate the sudden pressure change.
Slightly shorter but even heavier and more powerful were 2-6-6-6s built by Lima for the C&O and the Virginian between 1941 and 1948, which weighed 778,000 lb (353,000 kg) and could produce up to 6,900 horsepower (5,100 kW) at 45 mph (72 km/h).
[12] Although it had found early favor in Europe, especially on lightly engineered railways, the Mallet type was generally superseded by the Garratt locomotive by the mid-1920s.
The first generation of mallet used by Staatsspoorwegen (SS) was the 0-4-4-2T SS500 / BB10 class manufactured by Sächsische Maschinenfabrik (Hartmann) and Schwartzkopff, which came in 1899 for the mountainous track in West Java.
[13] In 1962, the Indonesian State Railways (DKA) ordered a series of 0-4-4-2Ts, basically an updated version of the earlier Dutch design, for the old Aceh tramway.
A number of the Union Pacific "Big Boys", are preserved, including one overlooking Omaha, Nebraska where UP is based.
In January 2014, Big Boy #4014 was removed from its museum ground parking track in Pomona, California, and hauled to Cheyenne, Wyoming, for restoration to operating condition; this was completed in May 2019.
It was taken out of service in October 2010 due to mechanical problems and retired from the Union Pacific's excursion program in January 2020.
"New as they were, the last C&O steam engines never got adequate maintenance, lengthening the list of work needed to bring 1309 back to life.
Several smaller logging-railroad Mallets have been restored to operating condition, including 2-6-6-2T Black Hills Central #110 in Hill City, South Dakota, 2-6-6-2T Clover Valley Lumber Company #4 in Sunol, California, and 2-4-4-2 Columbia River Belt Line #7 "Skookum" in Garibaldi, Oregon (#7 is currently based in Sunol with #4).
[citation needed] A number of Mallets were constructed for the Nordhausen Wernigerode Eisenbahn, now part of the Harz Narrow Gauge Railways system in Germany.
[22] ABPF-SC (Brazilian Association for Railroad Preservation – Santa Catarina branch) has restored a 2-6-6-2 Mallet to working order.
