Around 1910 it had become clear that the existing locomotives were no longer able to pull heavy freight trains over the Bavarian ramps at sufficient speed. Despite two pusher locomotives, trains could often hardly go more than 20 km/h, which could no longer be described as economical. Thus, Maffei developed a heavy tank locomotive in Mallet construction with two eight-coupled bogies. 

The resulting locomotive was the most powerful tank locomotive in Europe at the time. Since no carrying axles were necessary due to the low speeds on the ramps, the entire mass could be used as adhesive mass. It was able to pull 670 tonnes at 18 km/h alone on a gradient of two percent. At 2.5 percent it could still tow 540 tonnes, which was 100 tonnes more than two examples of the E 1.

A total of 15 of this locomotives were built in 1913 and 1914, and they proved themselves immediately. Both with a single pusher loco behind a freight train or with only one Gt 2x4/4 each as leading and pusher loco, significant increases in the speeds at the ramps were now made possible. After strong competition emerged with the Prussian T 20 in an 2-10-2T wheel arrangement, another ten Gt 2x4/4 were produced in 1922 and 1923. These were revised in many aspects to be even more powerful.

In 1925 and 1926 all engines were modified in order to bring both series to a more uniform and somewhat stronger level. The supplies and the total weight were increased, which now made more adhesive weight available. In addition, seven wheel sets instead of two were now sanded, which was a great advantage under difficult traction conditions. The Reichsbahn took over all 25 engines and classified them as class 96. After the end of World War II, 18 remained, two of which went to the Soviet occupation zone and the rest to the West. The Bundesbahn retired them as a minor class by 1948 and they remained in service with the Reichsbahn until 1954.

When the Clinchfield needed more 2-8-8-2 Mallets in 1923, they had two earlier classes to take as a basis: either the L-1 built by Baldwin in 1919 after their own design or the USRA standard design built by Baldwin in the same year that was called class L-2. They decided to order ten nearly exact USRA copies, which were now built by ALCO-Brooks and designated class L-3. They were used to haul heavy coal trains over the Appalachian mountains. One suffered a boiler explosion in 1948 and was scrapped. While the L-1 was retired by 1950, the L-3 and their L-2 sisters were only retired between 1950 and 1952.

In 1910, the DM&N received initially eight heavy Mallets of the 2-8-8-2 wheel arrangement, which they designated as class M and which largely corresponded to the class AB No. 600 of the Virginian. The intended area of operation was the ten miles from the docks at Duluth to Proctor at a gradient of 2.2 percent. Here the locomotive alone was to be able to pull or push 55 empty ore wagons uphill at 12 mph. 

They were still operated with saturated steam, but had a second feedwater heater between the high and low pressure cylinders in addition to the regular one. By default, no stoker was installed, instead two firemen were used. While the 55 cars were a great challenge for the two firemen on the grade, locomotives with a retrofitted stoker could transport up to 85 empty ore cars. In the years 1916 and 1917, two more locomotives each followed, which were designated as class M-1 and M-2. These had a superheater from the factory and were five feet shorter overall.

Of the eight class M locomotives, seven were fitted with a superheater in 1925 and all those that did not yet have a stoker were retrofitted with one. The last locomotive, number 207, was built in 1930 in a total rebuild. In the completely new boiler, the superheater had a significantly larger share, resulting in a heating surface that was almost 60 percent larger overall. The cylinders were now of the same size without compound working. In the same year, the four M-1 and M-2 received a similar boiler with the larger superheater and four simple cylinders, which made them the class M-2-S.

In the early years of the twentieth century, Mallet locomotives served only as an articulated solution for narrow-gauge railways. The L-1 of the Erie Railroad of 1907 was one of the first to show that very large freight locomotives could also be built with the Mallet principle. At the time it was commissioned, it was the largest and most powerful locomotive in the world. Since it was only intended as a pusher locomotive and did not have to reach high speeds, there was no need for leading or trailing axles. Thus, all eight axles were available as adhesive weight. The wheel arrangement was soon called “Angus”.

A Wooten firebox with a grate area of 100 square feet was used to maximize the energy yield from low-grade coal. In order to still ensure a good view for the engineer, the L-1 was built as a camelback engine and thus had the cab above the rearmost axle of the front bogie. It was the only camelback Mallet ever built.

Only three examples were built, which were used on the Delaware and Susquehanna divisions to push trains up a 1.3 percent incline. They were rebuilt to a more conventional form in 1921. The cab was moved to the rear and two carrying axles were added, whereby they now had the wheel arrangement 2-8-8-2.

On mountain lines such as Susquehanna Hill, the Erie Railroad had the problem that the 3,500 to 5,500 ton trains, each hauled by a Consolidation or Mikado locomotive in the flat, could only be brought over the grade with difficulty. Although they already used Mallet, often two helper locomotives were necessary. With the support of George R. Henderson of Baldwin, a triplex locomotive was built, which had three groups of eight drivers each. Only the prototype was built in 1914, christened “Matt H. Shay” after a well-known railroad employee.

The six cylinders were all of the same dimensions to enable compound action with two high pressure and four low pressure cylinders. The cylinders in the middle acted as high-pressure cylinders, with the exhaust steam from the right-hand cylinder being fed into the cylinders of the front truck and the exhaust steam from the left into the cylinders on the rear truck. Only the exhaust from the front low-pressure cylinders went into the smokebox, while the rear operated a feedwater heater in the tender.

During operation, it was quickly noticed that the boiler was not producing enough steam to reach significant speeds. Although the grate on the following two locomotives of 1916 was increased from 90 to 122 square feet, they too failed to produce sufficient steam. Even the large firebox with brick arch and combustion chamber could not improve this situation. As a result, the locomotives were only used as helpers, where average speeds were barely over 10 mph. 

The powered tender also proved problematic in practical use. It offered a smaller capacity than conventional tenders of the same size because the engine took up space and increased the empty weight. When the supplies were used up, the adhesive weight of the tender also dropped so much that it quickly began to slip. Since the other driver groups were now also suddenly subject to greater loads, they quickly lost grip. Thus, their number stagnated at three and the Virginian class XA remained the only other triplex locomotive that was built. A quadruplex locomotive already planned by Henderson was never built. The three P-1 were retired in 1929, 1931 and 1933.

In order to prevent individual chassis groups from slipping on slow freight trains, the Great Northern ordered a Mallet with unequal axle distribution. The front group consisted of a leading axle and three driving axles, while the rear group consisted of four driving axles. After ten locomotives from 1909, another 25 followed, which had a significantly larger tube heating area. Under the new scheme, they were designated class M-1.

They were converted to simple expansion to save complexity from 1926 and were designated class M-2. The rear cylinders remained slightly larger in order to be able to convert enough power for the additional driving axle. The existing feedwater heater was also removed. The locomotives had a choppy running that made speeds over 25 mph (40 km/h) difficult to run. 22 were scrapped from 1929 and their parts used for new O-7 class Mikados, while the remaining 13 partially survived until 1954.

The last generation in their series of 2-8-8-2 wheel arrangement Mallets, begun in 1910, was built by Norfolk & Western from 1930. While most other North American railroads had long since abandoned real Mallets and introduced ones with simple steam expansion instead the last series still had a compound engine. They were all made in the Roanoke workshops of Norfolk & Western and it all started with the Y4a as a development of the Y4 with an enlarged grate and significantly higher boiler pressure. Some optimizations were made to the internal piping that increased efficiency.

The first eight Y4a left the factory in 1930 and still had forged frames. On the ten Y5s from 1931, the frames were cast in one piece. The Y4as were also fitted with the new frames in 1940 and 1941. Also in 1940, 35 Y6s followed, in which the cylinders were now manufactured together with the frame as a cast part and had roller bearings on all axles. These were followed by another 15 Y6as, delivered in 1942.

Between 1948 and 1952, 30 examples of the Y6b followed, which would become the most modern Mallet locomotives in North America. They featured a larger firebox and a special mechanism that allowed them to run at all speeds with single steam expansion, increasing tractive effort at the expense of efficiency. The front bogie with the low-pressure cylinders was weighed down with several tons of lead to increase the adhesive weight. The diameter of the coupled wheels was one inch larger, and these were also retrofitted to the predecessors. In the last few years of service, the locomotives were equipped with technical aids that increased the starting tractive effort to almost 170,000 pounds or 755 kN.

All in all, the locomotives are considered to be the most powerful, commercially successful steam locomotives ever. Some earlier prototypes could muster even higher starting tractive effort, but the boiler no longer provided sufficient steam as the speed increased. However, the Y4a to Y6b could also haul heavy trains at up to 50 mph and at 25 mph with 13,500 tons on the hook achieved a drawbar power of 5,500 hp. Although this was surpassed by the Big Boy, this was designed for higher speeds and therefore had a lower starting tractive effort than the locomotives described here with single steam expansion. These large Mallets were retired by 1960 and today only Y6a number 2156 survives. It belongs to the Saint Louis Museum of Transportation and is not roadworthy.

In 1913, the Northern Pacific had its first superheated Mallets made for the operation of heavy, slow freight trains over the Cascades and in the Rocky Mountains. The ten locomotives came from ALCO-Schenectady and had the wheel arrangement 2-8-8-2. Four were coal-fired and destined for the Rocky Mountains, while the other six were oil-fired for the Seattle Division. A total of eleven more from ALCO Brooks followed in 1917 and 1920.

With cylinders measuring 26 and 40 inches in diameter, respectively, they developed more than 85,000 pounds of pulling power. However, these had such a high steam consumption that the highest power was already available at 10.6 mph. In operation, it had been shown that they could haul almost 30 percent more load as pilot compared to a class Z-1 locomotive while consuming the same amount of coal. In the cascades they pulled up to 2,400 short tons alone over most of the run, but were helped by another Mallet on the steeper sections with up to 2.2 percent.

In order to explore the extreme limits of the Mallet principle, the Atchison, Topeka & Santa Fe developed the class 3000 in 1911 with the wheel arrangement 2-10-10-2 and a weight of 309 short tons. The goal was to be able to pull 2,000-ton trains with just one locomotive over gradients of 1.2 percent in Arizona. In order to speed up development and production, the running gear for each of the ten examples was taken from two standard Baldwin 2-10-2 locomotives and combined into one locomotive. The existing cylinders were used as rear high-pressure cylinders and Baldwin had larger low-pressure cylinders made for the front bogie. A special development was the flat six-axle tender with a rounded rear section, which was called “Turtleback” and was intended to provide better rear visibility. It contained oil for firing.

A normal boiler of the same length as the class 3000 could not be realized and therefore only the rear half was used as a classic boiler barrel with combustion chamber and smoke tubes. The front half was used as a feedwater heater, superheater and reheater for the exhaust steam from the high-pressure cylinders. All in all, it was found that this boiler did not generate enough steam for the powerful engine and therefore sustained speeds of only 10 to 15 mph were possible.

In view of the low speeds, the locomotives were not used directly to pull heavy freight trains, but had to push other trains uphill. There they were mainly found in California on Cajon Pass and in the famous Tehachapi Loop. Nevertheless, the demolition of the first locomotives back to simple 2-10-2 locomotives began as early as 1915. When the last ones were dismantled in 1918, they had earned the nickname “magnificent failures”. Only the new design of the tender was considered successful and used in later Santa Fe locomotives.

In 1918, the Seaboard Air Line received 16 2-8-8-2 Mallets from ALCO-Richmond to move fast, heavy freights on their main line between Richmond, Virginia and Hamlet, North Carolina. After the wartime traffic had ended, it became clear that the SAL didn't have many uses for them since they were too heavy for many of their secondary lines and didn't fit the clearances in many places.

So all were sold to the B&O in 1920 and only two years later, they were rebuilt to class EL-6a. In this process, they got the wheel arrangement 2-8-8-0 by removing the trailing axle. This considerably increased the adhesive weight and at the same time they were simpled and got a higher boiler pressure, what also increased their starting tractive effort. In this form, they were used until 1954.