Mallet, Jules T. Anatole

Westwood claims that Jules Anatole Mallet was remarkable amongst late nineteenth century innovators in that he achieved a influential success both in compounding and in a method for articulating the driving wheelbase. The resulting Mallet articulated locomotive became especially popular in the USA, where it attained great size. Mallet's ideas on compounding inspired many subsequent designers to develop their own compound locomotives, some successfully, some very unsuccessfully.

Mallet was born at Carouge, near Geneva, in 1837, and studied and later taught engineering at the Paris Ecole Centrale des Arts et Manufactures. He first attracted attention in 1877, when the Bayonne-Biarritz Railway put two tank locomotives into service, designed according to Mallet's two-cylinder compound system with a single high-pressure cylinder passing its exhaust steam into a second, larger, low-pressure cylinder. The Biarritz locomotives worked well, but like subsequent two-cylinder compounds they tended to be unsteady at high speed, because one cylinder exerted more thrust than the other. Mallet was unable to interest any of the mainline railways in his idea. This lack of enthusiasm is not surprising when it is remembered that even after compounding had been adopted by many railways, it was never adopted by a majority. Those who rejected the idea almost always did so on the grounds that any fuel economies obtained from so-called double expansion were lost by the extra complication of compound machines. This criticism of compounding would be reinforced after superheated steam had shown another way of overcoming the basic problem that compounding attacked: that is the condensation of steam inside the cylinders which resulted from the fall in temperature as the steam expanded. Compounding broke the steam expansion into two parts, divided between two cylinders and thereby made it easier to cope with cylinder wall condensation. Superheating raised the steam temperature so that even after cooling it would remain higher than condensation temperature. Ideally, from the point of view of thermal efficiency, the most efficient machine would be one embodying both superheating and compounding, and many such machines were built in the twentieth century. In the 1870s, however, compounding seemed the only solution. Mallet believed he had a workable compound system, but could not persuade French engineers to try it.

However, the proliferation at that time of narrow-gauge light railways gave Mallet another avenue of approach. These lines required more powerful locomotives than their winding tracks could tolerate, and the only solution seemed to be some form of articulated locomotive. Two articulation systems were already fairly widely adopted. These were the Fairlie and the Meyer concepts, both of which embodied two pivoting engine units, supplied by steam through pipes with flexible joints. These flexible joints, so difficult to keep steamtight, were a weakness of these systems, and Mallet believed he had a solution in his own system of articulation, which he patented in 1884. Instead of two pivoting engine units, he had just one, placed beneath the smokebox. A second engine unit was at the rear, but this was non-pivoting. It was on this rigid rear unit that the boiler was fixed. For Mallet, the important feature of this layout was that it was a perfect setting for a compound system. Steam was taken first to the high-pressure cylinders of the rigid rear unit, and then piped to the cylinders of the leading pivoting unit for re-use at a lower pressure. In this way it was only the low- pressure steam which passed through the flexible steampipe joints, thereby easing the problem of steam leaks. The first such Mallet locomotive appeared in 1888, being built in Belgium for Paul Decauville. In 1889 Decauville's 60cm gauge line at the Paris Exhibition carried more than six million visitors and assured the continuing success of his enterprise. It also assured the future success of Mallet's compound articulated locomotive, for it was on this line that the first Mallet units made their debut.

The obvious success of these machines was followed by orders for similar narrow-gauge units from many railways, at first with the same 0-4-4-0T wheel arrangement but later in other versions. In the nineties the Mallet tank locomotive was joined by the Mallet tender locomotive in Switzerland and Germany.

In 1904 the Baltimore & Ohio Railroad introduced the Mallet concept to America, ordering an 0-6-6-0 from the American Locomotive Company for use on its Sand Patch incline over the Alleghenies. By 1911 more than five hundred Mallets had been built for US railroads. During the First World War the Virginian Railway brought the original Mallet concept to a peak so far as size was concerned, ordering 2-10-10-2 units whose low-pressure cylinders were 48in. in diameter. This Virginian design represented the virtual limit of size for the conventional Mallet locomotive. The overhang of the boiler at the front end on curves was excessive, and the low pressure cylinders were so large that it was impossible to design adequate valves for them, which meant that they worked efficiently only at low speeds and long cut-offs.. Moreover, the 4ft diameter cylinders were the biggest that could be accommodated on American railroads. For this reason most subsequent American Mallet locomotives were simples, not compounds. Later, American designers eliminated another fault which inhibited high-speed running with Mallet locomotives. This was the rough riding of the forward engine unit, which was only loosely attached to the main bulk of the locomotive.

Eventually Jabelman of the Union Pacific modified the articulation and applied a four-wheel leading truck to produce the Challenger 4-6-6-4, which could run up to 80 mile/h. This type was developed into the 4-8-8-4 Big Boys, which are regarded as the most powerful locomotives ever built. Although, at its peak of popularity, the Mallet locomotive was ordered by railways in many parts of the world, it was only in America that it retained its market up to the end of the age of steam.

The Garratt form of articulation, developed later, was technically superior, while many central European lines found that they did not really need articulated types. As for Anatole Mallet, it is said that he did not approve of the concept of the simple Mallet locomotive, as he had evolved his system of articulation as a means of promoting his compound system. In the twentieth century he was something of a grand old man of French engineering, regularly contributing comments on locomotive matters to the Memoires of the French society of civil engineers. He also designed the original locomotives for the Lartigue monorail system. Relatively little information has survived about Mallet the man, even though he was probably one of the three most important post-Stephenson locomotive engineers.

See: I. Vilain, Les Locomotives Articulees du Systeme Mallet dans le Monde (1969)
A. E. Durrant, The Mallet Locomotive (1974)
Journal de Geneve, 16 Nov. 1919.

Updated: 2003-02-12