On the Raton Pass between Colorado and New Mexico, the ATSF used pusher locomotives with a 2-10-0 wheel arrangement. Since there was no way to turn the locomotives on the summit, they had to run backwards down the mountain. In order to improve reverse running characteristics, some 2-10-0 locomotives with an additional trailing axle were ordered from Baldwin. This led to the Class 900 with the new 2-10-2 wheel arrangement, now known as the “Santa Fé”.

Propulsion was by four compound-acting cylinders arranged in tandem. The high-pressure cylinders were each directly in front of the low-pressure cylinders and worked together with them on the third coupled axle. To service the low-pressure cylinders, the high-pressure cylinders could be removed with an onboard crane. The third coupled axle was also the only one that didn't have wheel flanges. 

Original variant as tandem compound

The axle load of the first two driving axles and the leading axle was balanced with each other by means of a equalizing beam, as was the axle load of the rear three coupled axles and the trailing axle. Some locomotives received a special form of firebox developed by Henry W. Jacobs and Frank W. Shupert of Frisco Lines. This used perforated plates instead of staybolts and had advantages in terms of maintenance, but was not able to establish itself.

A total of 85 units of the class 900 were built. The first 40 of these were destined for Raton Pass and were coal fired. The rest were used on the line between Albuquerque and Arizona and were oil-fired. From 1905, 76 class 1600 locomotives were built, which were essentially identical to the class 900. However, since the frame had broken on this one, the frame was reinforced here, which increased the adhesive weight. However, different data for both classes could not be found.

From 1916 the locomotives of both classes were rebuilt, after which they were identical again. The boilers were equipped with a superheater and arch tubes in the boiler. Since the tandem engine required too much maintenance, it was replaced by a two-cylinder engine with simple expansion. Only the high-pressure cylinders were removed and the low-pressure cylinders enlarged.

The Santa Fe ordered 40 2-6-2 express locomotives in 1901, which had a driver diameter of 79 inches (2,007 mm) and a four-cylinder Vauclain compound engine. These formed classes 1000 and 1014 and were intended for routes such as the section between La Junta and Albuquerque, which had steep gradients of up to three percent. They had a large boiler, but only a comparatively small firebox. Three were also built with a Jacobs-Shupert firebox. 

In 1902 and 1903, a further 103 class 1050 locomotives were built. These had a driver diameter of only 69 inches (1,753 mm), which still ensured sufficient speed. Like their predecessors, they were also built by Baldwin. Between 1912 and 1924 the locomotives were rebuilt to simple two-cylinder engines. From 1918 they also received a superheater. The oldest locomotives also received smaller 69-inch wheels and some were converted to oil firing. They were retired between 1941 and 1956.

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.

The Santa Fe class 1480 consisted of 23 Atlantics, which they built in 1910 at their own factory in Albuquerque. They initially had a three-dome boiler, a Jacobs-Shupert firebox and a four-cylinder compound engine. The steam was taken from the rear dome, went past the sand dome into the front dome and from there via an external pipe into the reheater in the smokebox. Also noticeable was the long wheelbase between the second driving axle and the trailing axle. 

Their nickname among the crews was “Bull Moosers” and the area of operation was lighter express trains. Following the general trend of the time, 14 of the 23 were rebuilt from 1920 into superheated locomotives with two cylinders of the same size. The wheelbase between the second driving axle and the trailing axle was also shortened by more than three feet. The first was retired in 1926, the last in 1953.

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.

The class 3400 were the last Pacifics that the Santa Fe purchased. After many experiments had been attempted at the beginning of the century, these were now relatively simple machines with a simple two-cylinder engine. While the number of tubes in the boiler was based on the USRA Heavy Pacific, the firebox was only about the size of the Light Pacific. 

Between 1936 and 1947, 44 of the 50 locomotives underwent a major rebuild. In addition to an oil-fired boiler and combustion chamber, they received a new, integrally cast frame. There were also now Boxpok drivers with a diameter of 79 instead of 74 inches, which ran in roller bearings.

Since the rebuilt locomotives were significantly more powerful, their lives were extended. As they were increasingly less needed for express service, they were more and more often used with freight trains. The retirements finally took place between 1950 and 1955. Five have been preserved, of which only 3415 had been operational since 2009 and is currently waiting for its scheduled 15-year overhaul.

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.