After the KCS had been a friend of Mallets for a long time, they procured a total of ten locomotives with the wheel arrangement 2-10-4 in 1937. Five each were coal and oil fired. This was the first time a boiler of this size has been used with a boiler pressure of 310 psi or 21.4 bars. They achieved an indicated power of 4,825 hp and minus the mass of the locomotive and tender there was still 4,475 hp left at the draw bar. During the Second World War, they were also used to transport troops, and with their 70-inch driving wheels they were able to keep to the timetables of passenger trains. They were replaced by diesel locomotives in 1952 and 1953 and scrapped in 1954.

To meet the increased demand for heavy freight locomotives during the Second World War, the PRR turned to the C&O and used their T-1 as the basis for their J1. Because of this origin, they were given the nickname “War Babies” and were ultimately the largest 2-10-4 “Texas” class in terms of numbers with 125. They were built at the PRR's Juniata Works in Altoona.

With the booster on the trailing axle they achieved a starting tractive effort of 110,100 pounds (490 kN), while the boiler output of 5,644 hp was sufficient for higher speeds with heavy freight trains. Some of the locomotives had a frame cast in one piece. Depending on the source, these were the 60 members of subclass J1a.

A teething problem of the J1 was its rough running, which apparently arose from a mistake when taking over the blueprints from the C&O. This was soon fixed and at the same time the drivers were enlarged from 69 to 70 inches. With the switch to diesel, they became dispensable in their original role in the late 1940s. However, the PRR also found enough other tasks for them so that they were only retired between 1957 and 1959.

As early as 1919, the Santa Fe converted a class 3800 2-10-2 locomotive into a 2-10-4, creating the world's first locomotive with this wheel arrangement. This was done by exchanging the rear carrying axle for a bogie, while slightly increasing the boiler's heating surface. However, since the firebox was not enlarged, the gains in performance were limited and it remained a one-off. However, since the Texas & Pacific had built the first 2-10-4 in series from 1925, this type was called “Texas”. It was not until 1930 that the Santa Fé built engine number 5000 directly as 2-10-4, which soon became known as “Madame Queen”.

The boiler was designed to be significantly more powerful with a grate area of more than 120 square feet, three thermic syphons and a total heating surface increased by 34%. This made the locomotive significantly heavier and reached a maximum axle load of more than 76,000 pounds. The tender was also very large with a total weight of 375,000 pounds, which was carried on six axles. It held 20,000 gallons of water and 46,000 pounds of coal. Compared to its predecessor, it could tow 15% more weight in 9% less time with 17% less coal per ton-mile. It formed the basis for the classes 5001 and 5011 and remained in service just as long as these, i.e. until 1957.

As a direct development of locomotive number 5000, ten units were built in 1938, which were given the numbers 5001 to 5010. They differed from their predecessors primarily in the significantly larger driving wheels. These measured 74 inches in diameter and allowed the locomotives to run at up to 70 mph without any problems, although they were actually only intended as freight locomotives. The boilers had a slightly smaller total heating area, but the firebox area was larger and the boiler pressure had been increased from 300 to 310 psi. This resulted in a cylinder output of more than 6,000 hp, which almost compensated for the loss of traction due to the larger wheels.

No. 5004 on a factory photo

Ron Ziel, „American Locomotives 1858 to 1949”

The cylinders were cast in one piece with the frame, resulting in a single component weighing approximately 87,000 pounds. The first five locomotives got tenders with a lower curb weight, which now held 21,000 gallons of water and 52,000 pounds of coal and still only came loaded to 366,200 pounds. The last five locomotives were oil-fired, with tenders holding 20,000 gallons of water and 7,000 gallons of oil. Another development followed six years later in the form of the class 5011.

Although the Santa Fe was already in the midst of converting freight traffic from steam to diesel during World War II, it procured 25 more steam locomotives of the 2-10-4 wheel arrangement in 1944. They were all delivered with oil firing, got the numbers 5011 to 5035 and were also called “War Babies”. Despite a smaller firebox, the performance of the boiler could be kept almost the same. By distributing the weight differently, it was possible to increase the adhesive weight by four tons while the locomotive weight had fallen by four tons. Significantly heavier tenders with eight axles were used again, which held 24,500 gallons of water and 7,000 gallons of oil. This lifted the total weight of the locomotive and tender to just over the magic one million pound mark.

In 1944, measurements of the performance of the first locomotive in the batch were made, which led to sensational values. In one test, a five-mile grade of a constant 0.8 percent was climbed by a train of 94 cars weighing about 5,440 tons. From an initial speed of 56 mph, there was still 18 mph left at the top of the incline, where most other locomotives would have come to a standstill long ago. With these performances, the locomotives can certainly be described as the pinnacle of the single-frame steam locomotives, as they were only surpassed by articulated locomotives and more driving axles overall. They, too, made their last commercial journeys in 1957 and stood in reserve for another two years before almost all of them were scrapped. Four examples are still preserved today, none of which are operational.

The first locomotives that were built from the start with the 2-10-4 wheel arrangement were the ten members of the class I-1 of the Texas and Pacific from 1925. The goal in the development was to create a ten-coupled freight locomotive with a larger firebox to provide greater power at higher speeds. Since the Santa Fe had just rebuilt the first locomotive with this wheel arrangement from a different one, this wheel arrangement was now called “Texas”.

They had oil burning and a grate with an area of 100 square feet or 9.29 m². The area of the firebox was increased with a combustion chamber and two thermic syphons. There was also a booster in the trailing bogie and an Elesco feed water heater.

The ten I-1s had a driver diameter of 63.5 inches (1,613 mm) and a boiler pressure of 250 psi (17.25 bars). Between 1927 and 1929, four batches of 15 each were built, designated I-1a to I-1d. They now had a driver diameter of only 63 inches and a boiler pressure of 255 psi. They were also 4,000 pounds heavier, or another 5,000 pounds on the I-1d.

Although their service life was already coming to an end in 1948, three were equipped with Timken roller bearings on all axles and designated Class I-2. By the early 1950s all but the 610 and 638 were scrapped. While the 638 was also scrapped two years later, the 610 was preserved. It pulled the American Freedom Train in the 1970s and has not been operational since 1982. Today she can be found in Palestine, Texas.