The AA I was a prototype used to test ways of saving coal and water by using a lift axle. It was based on the B XI, which was built from 1892 both as a conventional saturated simple locomotive and with a compound engine. The 4-4-0 wheel arrangement of the predecessor was modified by replacing the second coupled axle with a trailing axle. For this purpose, the lift axle with small wheels was used between the lead bogie and the first coupled axle, which was driven by a second engine with two smaller cylinders.

With this configuration, it was possible under most conditions to drive with only one driving axle, thus saving coal and water. The lift axle was raised using a pressure cylinder because running along with the additional engine being turned would have required more power. If more power or traction was needed, the lift axle was lowered and some of the steam was fed to its cylinders. Although the locomotive with this system was economical and reliable on a day-to-day basis, the complicated engine required more maintenance than with conventional steam locomotives. So it was decided to continue to procure the B XI.

After an accident in 1907, the AA I was converted into a 4-4-0 locomotive, making it look like its predecessor. In contrast to this, it received a superheater. With this powerplant, it was able to last the longest of all Bavarian 4-4-0 engines in service with the Reichsbahn and was therefore not retired until 1933 as road number 16 861.

The “James Toleman” was designed by Frederick Charles Winby to test the installation of a long firebox between uncoupled axles. Each axle had its own cylinders, with the inner cylinders driving the first driving axle. Thus lone coupling rods could be avoided. The boiler was oval in cross-section to be fitted between large wheels. After attending the Columbian Exposition, it was given to the Milwaukee Road for trials. However, it quickly became apparent that it repeatedly had too little steam and parts regularly broke. Even a lengthy visit by Winby to the USA could not solve the problems that had arisen.

After the compound locomotives of the Webb type were apparently successful in Great Britain, the Ouest also ordered one locomotive for trials. Like the British locomotives, it had two small outer high-pressure cylinders driving the second driving axle and one large inner low-pressure cylinder driving the first driving axle. These two axles were not coupled to each other. The number 500 was rebuilt in the meantime, but was already retired in 1901.

The Greater Britain class of the London & North Western Railway consisted of ten 2-2-2-2 Webb compounds built in Crewe between 1892 and 1894. Like Francis William Webb's other 2-2-2-0 compounds, the two driving axles were not coupled. They had wheels with a diameter of 6 ft 10 inches or 2,083 mm.

Two high pressure cylinders on the outside drove the trailing driving axle, while the large single low pressure cylinder on the inside drove the leading one. Due to the combination with two high pressure and only one low pressure cylinder, they had diameters of 15 and 30 inches, respectively. The leading axle had a radially adjustable bearing, while the trailing one had half an inch of side play. Since Webb's successor George Whale preferred simple superheated locomotives, all were scrapped in 1906 and 1907.

The Q2 was created after the duplex prototype of the Q1 class with three driving axles at the front and two at the rear. This was only intended for freight transport and the number of driving axles in the first and second chassis groups was reversed compared to the Q1. As with the Q1, all the driving axles were mounted in the frame and the engines were divided to avoid too long connecting and coupling rods. 

Since the front chassis group had only two axles, the cylinders here were smaller than in the rear. In contrast to the Q1, the rear cylinders were still in front of the associated driving axles. Each group was prevented from slipping by an automatic apparatus. A booster was also installed in one axle of the trailing bogie. 

Overall, the Q2 is considered the largest non-articulated steam locomotive of all time. In static testing, one of the locomotives achieved an output of almost 8,000 hp. This made it possible to pull 125 loaded freight cars at a speed of 50 mph. On the other hand, maintenance was very complex.

In addition to the prototype, 25 production locomotives were built, making the Q2 the most successful duplex locomotive. However, the PRR fleet also included the class J1 with the 2-10-4 wheel arrangement which, with a nominal output of 5,650 hp, achieved almost the same starting tractive effort and was significantly easier to maintain. In addition, more and more diesel locomotives were added. This resulted in the Q2 being out of service by 1951.

In the search for a very powerful express locomotive to replace the K4 Pacifics, the PRR stumbled upon the problem of the excessive moving masses. The seven feet driving wheels would have resulted in extremely long coupling rods in a conventional, eight-coupled locomotive. These would not only have transmitted a strong hammer blow to the rails and locomotive, but could also have broken easily. The solution was the design of the duplex locomotives, which combined two separate steam engines on a common frame. To carry the heavy weight of the locomotive, a three-axle bogie was needed in front of and behind the driving axles. This allowed less than half the locomotive's weight to be used as adhesive weight, further increasing the tendency to slip.

Star designer Raymond Loewy provided the locomotive with streamlined Art Deco-style cladding and, including the eight-axle tender, reached a total length of more than 140 feet. Since they no longer fitted on the turntables, the PRR built special track triangles (wyes) for turning. Despite the bogies that could be moved laterally by 2.25 inches, it soon became clear that the S1 had a tendency to derail not only in many curves, but also in the wyes specially built for them. As a result, they could not be used on the entire line between Chicago and Pittsburgh as planned, but only on two-thirds of this line. In addition, if the drivers slipped, it was almost impossible for the engineer to determine which of the two engines was slipping and had to be braked accordingly. These were the main reasons why series production only came about with the T1 with smaller wheels and two-axle bogies.

With regard to the performance of the locomotive, there are different statements, since it was never run at the highest speeds together with dynamometer cars. What is undisputed is that they could haul the required 1,350-ton trains at 100 mph, giving an output of 7,200 hp. Higher speeds were often made to compensate for delays, although there were different observations regarding the maximum speed. Presumably more than 125 mph (201 km/h) were reached several times, but there are also indications in the range between 135 and 145 mph. Allegedly, the PRR once paid a fine when the locomotive reached 156 mph (251 km/h). Irrespective of this, the one-off, which could not be operated economically, was retired in 1946 and scrapped in 1949.

After the S1 built at the Altoona Works proved too large and of little practical use, Baldwin built the first two prototypes of the T1 for the PRR during World War II. The reason for purchasing such large express locomotives was that the trains had become heavier and heavier and the K4 class Pacifics used often had to be used double-headed. Like the S1, the T1 was of the duplex type and was streamlined by designer Raymond Loewy.

The two separate engine groups in one frame were necessary because large moving masses were created with a large wheel diameter and four coupled axles. These stressed the crew of the locomotive and exerted large impacts on the track. For this reason, conventional three- and four-cylinder engines were used in large express locomotives in other countries, which were unpopular in the USA due to the poor accessibility of the inner cylinders.

The special feature of the T1 was that two two-cylinder engines, each with two coupled axles, were located one behind the other in a common frame. The biggest disadvantage of the duplex engines was the increased tendency to slip. If one engine group started to slip, the crew could not tell which axles were slipping with the help of the exhaust blasts, and they had to be very sensitive with the throttle, especially with such powerful locomotives. For this reason, the rear bogie of the second prototype was designed as a switchable booster to increase the adhesive weight when starting.

When the construction of passenger steam locomotives was possible again after the end of the Second World War, the PRR ordered 50 production machines without boosters. They were significantly more suitable for everyday use than the S1 and still achieved an output of more than 6,000 hp. This enabled them to pull trains weighing around 1,000 tons at 100 mph without any problems and is said to have reached a regular speed of 120 mph to make up for delays. As with the T1, there are unconfirmed reports of runs in excess of 125 mph. The PRR's decision in 1948 to convert all high-value passenger trains to diesel had a negative impact on the service life of the T1. This was also be implemented in a timely manner, so that all T1s were already retired between 1949 and 1952 and subsequently scrapped.

The organization “The T1 Trust” was founded with the aim of building a working T1 from the ground up. Work began in 2014 and by 2020 large parts of the boiler, the driver's cab, parts of the fairing and two coupled wheels had already been completed. An eight-axle model was bought as a tender, which comes from an M1 and is to be provided with a streamlined fairing. After the planned completion in 2030, the locomotive should be used regularly and help to reconstruct the true performance of the originals. With a targeted maximum speed of 140 mph, the previous speed records for steam locomotives are to be broken. As of November 2023, the organization said it was 43.3 percent complete on the loco.