Although the Baltimore & Ohio wanted to buy diesel locomotives for freight traffic in the forties, they had to order steam locomotives due to the war restrictions. This was the birth of the class EM-1 with the wheel arrangement 2-8-8-4, called “Yellowstone”, being the last articulated of the B&O.

They were thoroughly modern locomotives with roller bearings on all axles including the tender. Thanks to five thermic syphons, arch tubes and a long combustion chamber, the firebox had an enormous surface of 756 square feet, larger than that of the Big Boy. With 117.5 square feet, the grate was smaller than that of the Big Boy that had to burn low-quality coal. Both lead drivers and the lead axle of the trailing bogie had some sideways play.

At the beginning, they operated in the Cumberland area with challenging grades. Services included everything from express freights to heavy coal trains, with occasional passenger trains. For the heaviest trains, they operated in double with another locomotive pushing at the rear. Later they came into the Pittsburgh area and were scrapped between 1957 and 1960.

After various Mallets with the wheel arrangement 2-6-6-2, the C&O had the class H-7 built with the wheel arrangement 2-8-8-2. In 1923 and 1924, 25 H-7s were delivered by ALCO and in 1926 20 H-7a by Baldwin followed, which had a slightly higher adhesive weight. Since, unlike their predecessors, they worked with simple expansion, they were called “Simple Simons”.

By 1935 they received a boiler pressure of now 215 instead of 205 psi and thermic syphons. The 632 square foot firebox heating area now included 145 square feet from the combustion chamber and 165 from the thermic syphons. They could haul a 9,500 short tons freight train 113 miles in five hours.

They were replaced on the flatter lines by the class T-1 2-10-4s and on hilly lines by the 2-6-6-6 “Alleghenies”. 30 of the 45 locomotives were later leased to the Union Pacific. The Richmond, Fredericksburg & Potomac also bought some locomotives.

In order to significantly increase speeds of heavy and fast freight trains, the C&O had the H-8 class developed by Lima. The inspiration came from the class A of the Norfolk & Western with the wheel arrangement 2-6-6-4. A three-axle trailing bogie ensured that an even larger firebox could be installed than on the existing locomotives. It was given relatively large drivers and steam heating so that it could pull passenger trains at 70 mph (113 km/h) if necessary.

The boiler was even larger than on the Big Boy and the locomotives were otherwise designed according to the latest standards. With an output of around 7,500 hp on the drawbar, the locomotives were among the most powerful steam locomotives ever. The installation of a booster in the bogie was rejected because even higher tractive forces were not required. There is some controversy regarding the actual weight of the H-8, as changes during development made it significantly heavier than originally planned. The first driving axle was loaded with 86,700 pounds or 39.3 t, which was the highest axle load of any steam locomotive.

The C&O named the wheel arrangement “Allegheny” after the mountain range that had to be overcome. Here two H-8 hauled up to 11,500 short tons. There they replaced the 2-8-8-2 class H-7 and enabled approximately twice the speed for heavy coal trains. On flatter routes they replaced the 2-10-4 T-1 class and could pull up to 13,500 short tons alone.

The first 45 H-8s were built during World War II, during which time they had to pull both urgent cargo and troop transports. The Virginian had eight AG class locomotives built in 1945, which were based on the H-8. In 1948 the C&O received another 15 H-8s, which were again similar to the AG. As early as 1952, the first H-8s were replaced by diesel locomotives. By 1956 they had all disappeared and today 1601 and 1604 remain preserved in static condition.

In order to achieve greater pulling power, the D&RGW further developed their Mallets from the L-95 and L-107 classes to the L-131 class with single steam expansion. With the wheel arrangement 2-8-8-2 and an axle load of over 70,000 pounds it was possible to build a boiler with one of the largest fireboxes at that time. The class designation indicates a starting tractive effort of 131,000 pounds, although the commonly used formula gives about 140,000 pounds. However, the D&RGW determined the practically achievable pulling force to be 131,800 pounds and then rounded this value down.

In 1927, ten machines were initially built at ALCO-Brooks. Their area of operation was in the area of the continental divide, where there was a steepest gradient of 1.42 percent on the east side and of 3 percent on the west side. The train ratings for one of these locomotives were 3,300 short tons on the less steep side and 1,400 short tons on the steeper side. The drivers apparently complained about excessive smoke production, which became a major problem, especially in tunnels.

L-131 on a factory photo

Denver Public Library Special Collections, GB-8096

When ten locomotives of another series were built in 1930, the dimensions of the power plant did not change. To distinguish them, this time the tractive effort was rounded up and it was called L-132. These were now built in Schenectady and received a new boiler design. Much of the tubes were replaced with the larger flues, which contained additional superheater elements. Like the L-131, they remained in service until the mid-1950s, when the conversion to diesel took place.

For heavy ore trains with up to 115 cars, the Duluth, Missabe & Iron Range needed more powerful locomotives, where the most difficult task was to pull the empty trains up the 2.2 percent steep ramp from the port in Duluth to the marshalling yard in Proctor. The Western Pacific class M-137 was taken as the basis. With a two-axle trailing bogie, the wheel arrangement was 2-8-8-4 "Yellowstone", which allowed for a larger firebox and a larger all-weather cab for the cold Minnesota winters.

In 1941 eight members of the M-3 class were built and in 1943 ten more M-4s. Since the DM&IR had less traffic volume in winter, some of the locomotives were leased out. They were particularly valued by the D&RGW, where they had to pull heavy freight trains over a 10,239 foot high pass over the Continental Divide. With the exception of one locomotive that had an accident, all remained in use until they were replaced by diesel locomotives between 1958 and 1963. Today three remain, all of which are no longer operational.

When purchasing more powerful locomotives for the Girardot-Tolima-Huila route, an example from Baldwin in the USA was supplied in addition to the British locomotive with the number 57. While the competitor was a Kitson-Meyer locomotive, Baldwin's locomotive belonged to a design developed in the USA.

In the meantime, locomotives with a chassis similar to a mallet had become established there for very heavy trains, but instead of the compound engine, they only had two large cylinders with simple steam expansion on both engine groups. Since these weren't real mallets, the term “articulated” was commonly used instead. Baldwin also carried out the design for Colombia in this design, but as a tank locomotive. In contrast to the competitor engine, this one only had the oil reservoir behind the driver's cab, while the water reservoirs were completely in front of it.

Since the British engine was reordered promptly and was to be given the number 58, Baldwin renumbered the machine to number 72 when it was commissioned. This was soon followed by a further renumbering to 171. After the locomotive had pulled freight and passenger trains for a period that could not be determined, it was destroyed when the boiler explosion. This was probably due to the fact that the water level was too low for the descent on the steep route and the roof of the firebox was no longer covered with water.

The Norfolk & Western class A consisted of a total of 43 simple articulated locomotives, which were suitable for different types of trains from slow coal trains to passenger trains. The trailing bogie allowed for a very large firebox, generating around 5,400 horsepower even without thermosiphons or water pipes. To reduce friction, roller bearings were installed on all axles, the last five even had roller bearings on the drive and coupling rods. 

The maximum load for slow freight trains on flatter routes was between 13,000 and 14,500 short tons. Express freight trains could be pulled up to 5,200 short tons and passenger trains could reach speeds of 70 mph and more. Because of the large water usage, old tenders were converted to water-only tenders, adding an additional 20,800 gallons to the 22,000 gallon capacity. With a tender and an additional water car, the locomotives weighed 1,233,000 pounds. Their service life ended between 1958 and 1959.

No. 1206 in 1937 in Columbus, Ohio

collection Taylor Rush

Today the number 1218 from construction year 1943 still exists, which was restored between 1985 and 1987. It was then the world's strongest pulling operational steam locomotive. With the termination of the Norfolk & Western steam program, however, it was parked again after only four years and is now waiting to be refurbished again.

The first Challenger locomotives, i.e. the wheel arrangement 4-6-6-4, were built in 1936. In addition to the well-known Challenger of the Union Pacific, this was the Z-6 class of the Great Northern. Since these had to burn lignite, the firebox had to be of enormous size to achieve the required output. The grate alone was 16 feet long, the entire firebox including the combustion chamber was 20 feet 6 inches long, 9 feet 6 inches wide and up to 7 feet 6 inches high. This made a grate area of 152.3 square feet and made the trailing bogie necessary to be able to carry the load.

Z-6 No. 5106 in September 1952 in Spokane, Washington

D. W. Eldridge / collection Taylor Rush

21 examples of the Z-6 were delivered by ALCO in Schenectady in 1936 and 1937. The first four examples still had friction bearings on the axles and were approximately 4,500 pounds or two tons heavier than the rest with Timken roller bearings, the weights of which are given in the table below. With a coupling wheel diameter of 69 inches, they could also pull heavy passenger trains if necessary.

Z-7 No. 5121 waiting for it's clear signal with a long freight train in November 1941 somewhere in Montana

W. R. McGee / collection Taylor Rush

In 1941, six heavier examples followed with slightly different boiler dimensions, one inch larger drivers and a boiler pressure increased by 10 psi, which formed the Z-7 class. Instead of the six-axle tender with a water capacity of 22,000 gallons, seven-axle 25,000-gallon tenders were used. In the years 1943 and 1944, 20 almost identical Z-8 were manufactured, but because of the wartime conditions, many of the high-strength steels had to be replaced with conventional, heavier steels. This added about 1,000 pounds to the mass.

The Z-8s were the last steam locomotives procured by the Northern Pacific. Because they were fired with lignite, they consumed gigantic amounts of fuel. On the 65 mile route from Townsend to Bozeman, Montana with a constant incline, there were reports of coal consumption of 831 pounds per mile. The relatively young locomotives were retired between 1954 and 1959. The Spokane, Portland & Seattle Railway procured almost identical locomotives, namely six Z-6 and two Z-8 with oil firing.

The first 2-6-6-4 articulated locomotive ever built was the class J-1 of the Pittsburgh & West Virginia. They were created in 1934 by Baldwin to surpass the power of existing 2-6-6-2 locomotives. In contrast to later 2-6-6-4s, the P&WV didn't need them for high power at high speed, but for heavy medium-speed freight trains in hilly terrain.

The three locomotives of class J-1 had drivers of only 63 inches (1,600 mm) and a two-axle booster in the tender that added an additional 16,000 pounds of starting tractive effort. The trailing truck allowed a Belpaire firebox with 499 square feet (46.4 m²) of heating surface and a grate of 102.3 square feet (9.5 m²).

Later the booster in the tender was removed and the drivers were enlarged to 64 inches. In 1937, four additional J-2 class locomotives were built without boosters. Even though they were not built for high speeds, they are reported to have reached speeds of 50 or 60 mph. Their retirement took place in 1953.