In 1938, the Atlantic Coast Line had Baldwin build twelve 4-8-4 Class R-1 locomotives, also known as Class 1800 because of their numbers, for express train service between Richmond, Virginia and Jacksonville, Florida. Their design was considered very successful and the boiler in particular exceeded many expectations. They pulled 1,500-ton trains of 20 cars and were even able to accelerate them significantly faster than the manufacturer had initially calculated.

Of note, the direct heating area of the actual firebox was increased from 272 square feet to 568 square feet  using a combustion chamber and thermic syphons. The back pressure of the cylinders was also very low, especially in the upper speed range, and allowed greater power output. The only known problem concerned the mass balancing, which was initially calculated incorrectly by Baldwin and still had to be improved.

After the adjustments to the mass balancing, the locomotives were approved for 90 mph and are said to have regularly reached 100 mph. When they later had to pull express freight trains, the maximum train load was set at 6,200 tons. Their service ended between 1951 and 1952.

For mixed service in front of freight, passenger and express trains, the Canadian National procured locomotives with the wheel arrangement 4-8-4 for the first time in 1927, known in English-speaking countries as “Northern”. They followed the “superpower” philosophy popular in North America at the time and boasted a grate area of more than 80 square feet thanks to the rear bogie. The heating surface of the firebox was maximized with thermosiphons, fire bricks and water pipes. What was special about the rear bogie was that the wheels on the two axles had very different diameters of 34 and 48 inches.

The first delivery from 1927 included 20 units each from the Canadian Locomotive Company and the Montreal Locomotive Works, which were designated as class U-2a and U-2b. The frame of both series was partly made of vanadium steel and nickel steel. Some had a booster on a rear bogie axle that provided an additional starting tractive effort of 10,900 pounds. ALCO built ten more, almost identical locomotives for the Grand Trunk Western.

From 1929 a new version was produced in which the relationship between the flues and tubes in the boiler differed from the first series. The frames were now only made of nickel steel. Instead of the spoked wheels, the lighter boxpok wheels were now used. The first 20 examples of this design came from Montreal and were designated U-2c. From there came five more as U-2d in 1936 and 15 as U-2e in 1940. Also in 1940, ten U-2fs arrived from the Canadian Locomotive Company.

U-2d No. 6161. The Boxpok wheels can be seen in comparison with the U-2a

collection Taylor Rush

The last version had a smaller tube heating surface than the second version and therefore a larger superheater. The first 35 were completed in 1943 by the Montreal Locomotive Works as the U-2g. Later that year and in 1944, another 30 were made at the same plant, known as the U-2h.

One example still in existence today is U-2g No. 6213. It was used until 1959 and is now owned by the City of Toronto. It has been displayed at Exhibition Place and has been cared for by the Toronto Locomotive Preservation Society over the years. In 2009 she was transferred to a roundhouse, where she was cosmetically prepared in 2019. In its current condition, it is quite conceivable that it could be restored to operational condition in the future.

Another U-2g that still exists is number 6200 in Ottawa. This was recently rebuilt so that it now can be towed. Number 6218 sits in Fort Erie, Ontario but hasn't been restored yet.

The first 65 class R-65 Northerns of the Rock Island were delivered by ALCO-Schenectady in 1929 and 1930. They had a driver diameter of 69 inches and were primarily intended for service with freight trains. Some later received 74 inch wheels to be more suitable for passenger trains and were renamed the R-67 class as a result. In the years 1944 and 1946 further R-67b were built, which had larger wheels from the start. Although they had a higher boiler pressure and a larger firebox, they had a smaller tube heating area and a significantly smaller superheater. The 1944 batch were oil fired while the rest were coal fired.

The Lackawanna was one of the first US railroads to adopt the 4-8-4 wheel arrangement. In contrast to the Northern Pacific, these were not called “Northern” here, but “Poconos”. They were seen here as the successor to the 4-8-2 with a larger firebox. Thanks to thermic siphons and arch tubes, they came up with a firebox heating surface of 493 square feet. With a driver diameter of 77 inches, they were clearly intended for express trains, while a diameter of only 73 inches was chosen at the Northern Pacific. The later Poconos of the Lackawanna also had smaller wheels.

When the French State Railways were already planning the complete conversion to electric and diesel locomotives, André Chapelon designed an express steam locomotive with an output of 4,000 hp at the drawbar. As a basis, he used the Est 241-101, which had not made it into series production, and modified it accordingly. He converted the simple three-cylinder engine into a compound engine and provided the locomotive with optimized steam pipes and a Kylchap exhaust system.

The result was the most powerful steam locomotive ever built in Europe, producing between 5,230 and 5,430 hp (3,900 and 4,048 kW), depending on the source. While the original locomotive only had 2,550 hp on the drawbar, over 4,000 hp were now achieved and express trains heavier than 1,000 tonnes could be pulled. Up to 158 km/h were reached on test runs and approval was given for 130 km/h. The machine was put into service in 1951, but was only used sporadically due to the decision to electrify the network and was scrapped in 1961.

For particularly heavy express trains on mountainous routes, two prototypes of the class 06 were built in 1939 as the only German steam locomotive with a 4-8-4 wheel arrangement. They got the boiler of the class 45 and streamlined cladding. As with the 45, the boiler with the overly long tubes caused problems. There were also occasional derailments in narrow switches. Due to the Second World War, there was no series production and after the end of the war, no revised machines were built due to a lack of demand.

Compared to the S-1, the locomotives of the S-2 class received larger driving wheels with a diameter of 80 inches, as they were only intended for express trains. They also burned oil and now had cylinders measuring 29 by 29 inches. They showed up more than 50,000 pounds less on the scales thanks to a smaller boiler. Since the adhesive weight was also reduced as a result, the locomotives slipped more quickly when starting and had to be operated with great care. Nevertheless, from 1949 they were no longer needed for express trains such as the “Empire Builder” and also had to pull freight trains.

The Lehigh Valley used the 4-8-4, which they dubbed “Wyoming” after the Wyoming Valley in Pennsylvania, primarily for freight trains. After one prototype each from Baldwin and ALCO, ten more were procured as T-1a and T-2a. These had the most powerful booster of all US steam locomotives in the rear bogie. The T-3 was built by Baldwin from 1934 and was also suitable for passenger trains with larger wheels. The T-2b first emerged during World War II and was derived from the T-2a. Due to the lack of material, however, this was less refined and thus weighed more heavily.

Since the three Class G Garratts procured in 1928 could not convince due to various problems, a new, powerful locomotive for heavy passenger and freight trains was needed for the mountainous North Island Main Trunk Railway. In order to be able to avoid further problems with articulated locomotives, a heavy locomotive with a 4-8-2 wheel arrangement was initially planned. A major problem with the limited loading gauge of the Cape gauge routes was getting the most powerful locomotive possible to operate under these conditions. The locomotives were designated class K after the previous class of the same designation, supplied from the USA in 1877, was phased out towards the end of the 1920's.

The design that finally came to fruition had a boiler of the maximum practicable diameter. The trailing axle was replaced with a bogie so that a large firebox could also be carried. In order to be able to arrange the largest possible cylinders within the loading gauge, they were designed with a diameter of just 20 inches and a long stroke of 26 inches. The Heusinger valve gear initially caused problems due to the excessively long piston travel, but these could be remedied by reducing the travel from eight to 7.25 inches.

The total number of 30 engines replaced mainly the 4-8-2 Class X locomotives. They performed very well with heavy trains and were mostly converted to oil firing after the Second World War due to the lack of coal. The changes made were carried out in parallel with their successors of the K^A class. Since there were frequent frame fractures in the area of the firebox in the K class, new frames of the same design as in the K^A were installed.

Although the switch to diesel traction took place in the mid-fifties, several examples of the class were still being overhauled at that time. All engines were finally decommissioned between 1964 and 1967. A total of three examples were preserved, two of which were last used as stationary steam generators. One of these two engines, as well as one that was recently exhibited in a museum, were selected for a renewed refurbishment in 2013 and 2014 respectively.