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The mallet is a semi-articulated locomotive with two driver units. The leading driver unit  swivels at a center point between the cylinders of the trailing driver unit. The trailing driver unit is fixed in a conventional locomotive frame not allowing any radial adjustment. Cylinders are always front facing. Buffing and draw gear are attached to the truck.

Another typical characteristic of the mallet is the principle of compounding.


When compounding the steam from the boiler is used twice to power the locomotive. Steam is fed directly from the boiler into the rear cylinders (orange), and the expanded steam is fed from there to the front cylinders (blue). This principle saves up to 40% on fuel consumption.



Anatole Mallet, a French engineer, designed his articulation system in the eighties of the nineteenth century. It was more or less a side product of his ambition to find a practical use for compounding. His patent was granted in 1885. The first mallets entered service in 1888.

Initially Mallet locomotives were built mostly in Europe and were mainly of the x-4-4-x type. Mallets were mainly used on narrow gauge line providing locomotives with four or six coupled axles without excessive wear of rails or wheels. When Gölsdorfs sideways moving axle was introduced rigid frame locomotives with multi coupled axles, up to as many as five or even six, became feasible, bringing development and production of the mallet in Europe to a halt by 1910.

A few exceptions were there however.

  • The German Baureihe 96-0 was built as late as 1922, being the only 0-8-8-0 tank mallet in the world, and the only eight coupler in Europe.
  • In Hungary development continued until as late as 1920. These developments are little known but very interesting. The largest mallets in Europe (that is loco including tender) have been built there. Contrary to many European mallets they were wide spread and aesthetically designed. Advocates of the Hungarian mallet claim that these engines form the pinnacle of European mallet design. I wholeheartedly concede to that claim. Take a look at to see for yourself.


The mallet was introduced in the in 1904 at a time when the decline in Europe had already begun. The continuous hunger for power on American railroad forced designers to ever larger concepts surpassing the possibilities of the rigid frame locomotive even when applying the sideways moving axles. Thus in the the concept of the mallet was readily adopted and continued. Size, weight and power soared to unsurpassed heights. For the largest designs compounding was abandoned in favor of the raw power of four cylinders directly fed with fresh steam from the boiler.

Final years

As mentioned earlier, mallet building largely came to a halt in Europe by 1910. By 1940 most of them were history.

In the the transition to diesel and electric traction caused the mallet to disappear. They were developed until as late as 1953 (Y6, Norfolk & Western) but all post war development proved to be short lived. Most steam traction had ceased by 1959.


There is an extensive array of footage available on Youtube, impossible to list it all, but I picked this one. It shows the tpical slow haul operation of the American articulated



Triplex mallets

A Mallet triplex has third driving unit trailing the fixed drive unit. The Erie Railroad had three Triplex 2-8-8-8-2 engine built in 1916. They were used as pushers on the Susquehanna incline. It proved difficult however to generate a large enough supply of steam to feed the cylinders. After withdrawal they were disassembled and used to build one ordinary mallet and one normal rigid frame locomotive.

One other Triplex, a 2-8-8-8-4, was ever built for the Virginian railway which didn't succeed either and was soon reconstructed to a normal mallet.

Mallets with an articulated boiler

If you think you've seen the funniest things of railroading, try this one

References and links


Mallets in the tall timber A most interesting site about logging mallets

Mallet Articulated Locomotives

Extensive description of the mallet principle
Mallet Articulated Locomotives From the same site, different description, very worthwhile reading.
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Specific Types

Mallets on the Rhätische Bahn  
Southern Pacific Articulated Steam Locomotives Complete overview of 282 units
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