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.

One of two classes of 4-6-6-4 “Challenger” locomotives ordered by the DRGW was the L-105 introduced in 1938. It was designed for mixed service on grades in the Rocky Mountains with up to three percent and sharp curves. For these long grades, it had a huge sandbox mounted in front of the smoke box. The 105 in the class designation denoted the starting tractive effort in thousands of pounds. 

The first ten were delivered by Baldwin in 1938 and got the numbers 3700 to 3709. In 1942, five more were ordered which got the numbers 3710 to 3714. They had the same basic dimensions, but got fewer tubes and flues. A typical line the L-105 was running on went over Soldier Summit in Utah. There they hauled 2,750 tons and were assisted by helpers on the steepest portions. The town called Helper was one of those places where the helpers were coupled to the train. They were retired and scrapped between 1951 and 1957.

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.

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.

In the twenties, the Great Northern needed powerful freight locomotives for the more than 900 mile-long stretch from Minot, North Dakota through Montana to Spokane, Washington. They needed lots of pulling power for the grades over Marias Pass, but enough speed for the long distance. The first four locomotives of class R-1 were delivered by Baldwin in 1925. They had the 2-8-8-2 wheel arrangement, four equal cylinders of 28 by 32 inches (711 by 813 mm) and drivers of 63 inches (1,600 mm).

At their boiler pressure of 210 psi, this resulted in a starting tractive effort of 142,165 pounds when using the usual formula. But since the Great Northern used a coefficient of .765 instead of .85, they only quoted a value of 127,950 pounds. The six-axle Vanderbuilt tender carried 16,800 gallons of water and 5,800 gallons of oil. Together, the locomotive and tender reached a total weight of nearly 450 tons.

Based on the first four R-1, ten more were built in the GNR'S own Hillyard shops in 1927 and 1928. But these were not only heavier, but had their pressure raised by 15 psi and got a bigger firebox heating surface. While the Baldwin locomotives only had a combustion chamber, the new ones also got arch tubes which added 64 square feet. Also the older ones later got the arch tubes.

Since their operator still needed more power, they built 16 of the class R-2 in 1929 and 1930. The water capacity of the tender was increased to 22,000 gallons. At a total weight with tender of more than 500 tons, they were the biggest US locomotives built west of the Mississippi. The boiler pressure was again increased by 15 psi, the grate was enlarged and the superheater was made much larger.

By 1946 they had been upgraded by increasing the firebox heating surface by 142 square feet through the addition of circulators and by an increased weight that led to a 24 tons higher adhesive weight. The combined surface of firebox, tubes, flues and superheater now reached a value of 11,349 square feet (1,054 m²). A special measure was the reduction of the diameter of only the front cylinders by two inches to match the steam consumption with the boiler.

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 thermic syphons or water tubes. To reduce friction, roller bearings were installed on all axles, the last five even had roller bearings on the connecting and coupling rods. 

The maximum load for slow freight trains on flatter lines was between 13,000 and 14,500 short tons. They could haul express freight trains up to 5,200 short tons and with passenger trains they 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 of the regular tender. With a tender and an additional water car, the locomotives weighed 1,233,000 pounds. Their service life ended between 1958 and 1959.

Today number 1218 built in 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 stored again after only four years. Now it is waiting to be refurbished again, although this is rather unlikely to happen.

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 Northern Pacific. 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 two-axle trailing bogie necessary to be able to carry the load.

21 members of the class Z-6 were delivered by ALCO-Schenectady in 1936 and 1937. The first four 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 driver diameter of 69 inches, they could also haul heavy passenger trains if necessary.

In 1941, six heavier locomotives followed with slightly different boiler dimensions, drivers one inch larger and a boiler pressure increased by 10 psi, which formed the class Z-7. 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 weight.

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 line 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.