The name “Old Maude” refers to a mule in a comic, but also the first large Mallet in the USA built for standard gauge. Its job was as a helper for heavy trains on inclines, what didn't require high speeds. This meant that she did not require any carrying axles, meaning that all six axles were available for traction. The top speed of 21 mph or 34 km/h was completely sufficient for her tasks. Although the locomotive, which was initially designated class O and later renamed DD1, did not have any sisters, it remained in service until 1938.

The Gts 2x3/3 was a heavy narrow-gauge locomotive for a gauge of 1,000 mm in Mallet construction and with six coupled axles. In fact, only one of the 20 locomotives that Henschel built for the Heeresfeldbahn during the First World War carried this designation. This one locomotive was bought by the Bavarian State Railways after the end of the war and used on the Eichstätt-Kinding route.

In addition to the high weight of more than 50 tonnes, the locomotives were characterized by the fact that, in contrast to many narrow-gauge locomotives of the time, they already had a superheated steam engine. With the relatively large reserves of 1.5 tonnes of coal and 4.5 cubic meters of water, it was possible to run for longer periods without refilling. In order to further improve the curve running characteristics of the chassis, the middle axle of each of the two chassis groups could be shifted 15 mm to the sides.

The use of the 20 built locomotives at the Heeresfeldbahn explains the fact that they were scattered over several countries after the end of the war. Some were also used on the Hijaz railway from Damascus to Medina. In the post-war period, the example taken over by Bavaria was used on the aforementioned route and was designated 99 201 by the Reichsbahn from 1925. After the line was converted to standard gauge in 1934, the locomotive became superfluous and was scrapped. In the years 1925 and 1928, a locomotives was built for two narrow-gauge railways in Baden. These were only designed as saturated engines and remained in use until 1957 and 1967 respectively. One of these was later used in Switzerland and can now be seen in the museum there.

The class N of the Burma Railways designated 17 Mallets built by North British in four batches between 1911 and 1921. With an 0-6-6-0 wheel arrangement, they had all weight available for traction. Nevertheless, they were not only intended for freight service, but also for passenger service.

Their area of operation was the Lashio branch and the Southern Shan states. Even though they had a huge pulling power, they had to be assisted by 2-6-2T locomotives on steeper sections. In 1924 Beardmore built five superheated Ns class locomotives and rebuilt the 17 others in the same form. It didn't take long until they were superseded by the Garratts starting with the GA.I.

The challenges for locomotives on the Gotthard railway were not only the incline of 2.7 percent, but also the spiral tunnels with a radius of 300 meters. This required a high tractive force and at the same time made the adhesion conditions more difficult. In order to be able to dispense with the previously necessary one or two auxiliary locomotives per train, eight-coupled locomotives or those with articulated chassis should be procured. In 1890 Maffei in Munich delivered locomotive number 151 to test the Mallet design.

The tank locomotive had six coupled axles and no carrying axles and was therefore given the type designation Ed 2x3/3. All cylinders acted on the respective third coupled axle. Due to the short distance between the wheels, only the first and last axle of each running gear group could be braked from the outside, otherwise there would have been no space for the brake pads between the wheels. Since enough space had to be planned for the water tanks on the sides of the boiler, the boiler turned out to be relatively small for a locomotive with a service weight of 87 tonnes.

Since a high steaming capacity was required for the long, continuous incline, the Ed 2x3/3 quickly reached its limits. Although the engine offered sufficient traction and was able to convert a lot of power, the boiler was too small in comparison. Calling up full power for a short time was also difficult because the maximum coupler force could be exceeded. Thus the maximum train weight of the Ed 2x3/3 on the Gotthard railway was only 200 tonnes, while the existing D 4/4 could pull 175 tonnes. The Mallet thus remained a one-off while further, reinforced batches of the D 4/4 were purchased.

The first Mallet with the 2-6-6-2 wheel arrangement was the L-1 class of the Great Northern. They were created in 1906 by adding a leading and a trailing axle to the 0-6-6-0 in order to improve running in curves. They were used as helpers in the Cascades. After the first five, 20 more followed in 1908 after the L-2, which had been delivered in the meantime, turned out to be too light. The class Z of the Northern Pacific was identical to these.

Although they had a Belpaire firebox, its heating surface was very small compared to the total heating surface. At least two L-1s received a superheater around 1915, and they also received significantly larger high-pressure cylinders. However, as early as 1925 and 1926, all L-1s were rebuilt to class O-6 Mikados, which remained in use until the mid-1950s.

The Santa Fe started in 1910 with one of the few attempts to develop a large mallet with large coupled wheels for use in front of passenger trains. The large wheel diameter resulted in a considerable overall length, which made special demands on flexibility in curves. Since the long boiler was an obstacle, the idea of a divided boiler came up.

Detail of the connection using steel rings

The locomotive with the number 1157 was assembled from the parts of two former Prairie locomotives (2-6-2), that now had the wheel arrangement 2-6-6-2. The greatest difficulty lay in connecting the two halves of the boiler in a flexible and tight manner. The water space between the two halves of the boiler was separated, while the hot smoke, complete with sparks and individual lumps of slag, passed from the rear to the front part of the boiler.

Class 1170 with ball joint

Locomotive Magazine, May 1911

The rear part of the boiler produced the steam and superheated it before it entered the rear high-pressure cylinders. The exhaust steam then entered the front part of the boiler where it was reheated and then sent to the front low pressure cylinders. The front part also served as a feedwater heater. The connection between the halves of the boiler was made by means of fifty steel rings nested alternately within each other. In operation, slag chunks often wedged themselves between the rings, which then burst in the next curve. This connection was adapted in later locomotives and a variant as a ball joint was also created.

Although it was not possible to find a completely satisfactory solution for the connection between the boiler halves, twelve more engines of this type were built in 1910 as classes 1158 and 1160. At the same time, 27 class 1170 units with a very long, one-piece boiler were built. Despite these circumstances, the Santa Fe had 24 examples of a heavier version with a two-piece boiler built in 1911. These were given the numbers from 3300 to 3323 and were thus referred to as Class 3300. Due to the continuing problems with these locomotives, they were later forced to either scrap them or dismantle them for the construction of Prairie locomotives as early as the 1920s.