In World War II the Southern Railway, usually making most of its revenue with commuter traffic, found itself on the front line. So they urgently needed a goods locomotive that could be built cheaply and quickly, saved strategic resources and was powerful. Oliver Bulleid decided to build a locomotive with the traditional 0-6-0 wheel arrangement, but more powerful than all others. He also removed all parts which were not essential for operation, but incorporated some innovations in the basic structure.

The most prominent feature of the Q1 was the boiler that had plain and flat side walls, which resulted from the fiberglass insulation and enabled them to use coach washers. The wheels with a diameter of 5 ft 1 in were of the Bulleid Firth Brown type also found on the Pacifics of the same designer. The cylinders were on the inside and had generous steam passages. The chimney was fitted with a five-nozzle blast pipe. All in all, they were still so light that they could be used on almost the entire Southern network.

20 locomotives each were built at Brighton and at Ashford. They were the most powerful 0-6-0 ever built in Britain and were assigned power class 5F when taken over by British Railways in 1948. Due to their unconventional look, they got several nicknames like “Ugly Ducklings”, “Coffee Pots” or “Frankensteins”. Nevertheless, they fulfilled their tasks and even hauled passenger trains on secondary lines. They were withdrawn between 1963 and 1966 and today only the first locomotive numbered C1 is preserved. It ran for the last time in 2000 and is now located in the NRM at York.

The Southern Railway class U were 2-6-0 passenger locomotives which were created by Maunsell when the class K 2-6-4T were found out to be prone to derailment. So the 20 first locomotives of the class U were rebuilt from the class K at Brighton and Eastleigh, while 30 more were built between 1928 and 1931 at Brighton and Ashford. They were influenced by the GWR class 4300 and SECR class K and had a tapered boiler.

With the class U, it was possible to withdraw some old 4-4-0 locomotives. Due to their low axle load, they could appear suddenly in nearly all corners of the Southern network, so they earned the nickname “U-boats”. The long-travel valves easily allowed speeds of 70 mph or more. British Railways got all 50 in 1948 and withdrew them between 1962 and 1966. Four were preserved and operational again at some time, but today only 31806 is operational.

The Southern needed a fast passenger locomotive for lines of medium importance, which allowed relatively high axle loads, but had a smaller loading gauge or shorter turntables. Since no locomotives with a 4-6-0 wheel arrangement could be used here, a locomotive with a 4-4-0 wheel arrangement that was as powerful as possible had to be designed. A higher axle load was also accepted and the Hastings line was rebuilt accordingly. The new locomotive was the last one in Great Britain with this wheel arrangement.

Maunsell took the Lord Nelson class as a model and designed a scaled-down version of it. Since its Belpaire firebox did not allow a cab that fitted into the loading gauge, a different boiler had to be used. This was found in the King Arthur class with a round-topped firebox. The firebox was adopted unchanged and the boiler barrel was shortened. With three cylinders, a starting tractive effort was achieved that was on par with the King Arthur.

The total of 40 locomotives were named after public schools, which is where the name “Schools class” came from. They were considered a very successful design right from the start and were the most powerful 4-4-0 locomotives in Europe. Despite the smoother power development of the three cylinders, they tended to slip when starting due to the high tractive effort. However, they also performed well with poor coal. A locomotive made the run from London-Waterloo to Southampton with a 510 ton train at an average of 51 mph (98 km/h). The highest recorded speed is 95 mph (153 km/h).

Over time, the locomotives received several improvements. From the second production batch smoke deflectors were installed and retrofitted to the first batch. 20 units later received a Lemaître blastpipe. Since this did not prove its worth, no more were rebuilt. Under British Railways, they were also used in the Western Region and Wales. They were soon replaced by diesel-electric railcars and retired between 1961 and 1962. Today three are preserved, all of which have run in reservation.

During the Second World War, the Southern Railway realized that it urgently needed new and more powerful express locomotives. These were primarily intended to be used in front of the increasingly heavier boat trains between London and the Channel coast. After there were initially considerations regarding the optimal wheel arrangement and eight-coupled machines were also considered, Oliver Bulleid decided on a Pacific. 

Since there were restrictions on the construction of express locomotives due to the war, the new design was officially classified for mixed service and was designed so that it could also be used in front of freight trains if necessary. The driving wheel diameter of 6 ft 2 in (1,880 mm) was a compromise between tractive power and top speed. The three cylinders also promised starting without much slip.

A batch of ten locomotives was ordered in each of 1941 and 1944. A third order for another ten was placed a few months before nationalization and only built for British Railways in 1949. Better materials than those used in the war could now be used, which reduced the weight. The class was initially designated 21C1 and named “Merchant Navy”. The names of the individual locomotives came from merchant navy fleets that were used in the North Atlantic during World War II. A short time later, the smaller sister class called “West Country”/“Battle of Britain” was created. Both were therefore also known as “Bulleid Heavy Pacific” and “Bulleid Light Pacific”.

The locomotives immediately attracted attention with their casing, which earned them the nickname “Spam Cans”, after the canned meat known in Britain. However, the fairing was not used to reduce drag, but rather to divert smoke and make the locomotives' exterior smoother for cleaning in carriage washers. It soon became clear that the crews' visibility was often limited by the smoke.

The boiler was characterized by a high pressure of 280 psi (19.3 bars), as well as a combustion chamber and thermic syphons and had a welded firebox. The driving wheels and those of the tender were a special type of disc wheel that Bulleid had developed. For reasons of space, he also developed a special type of valve gear. Each cylinder had miniaturized Walschaert gear that ran in an oil bath and was driven by a camshaft, which in turn was turned by the driving axle via chains.

Since the casing and the chain drive turned out to be problematic, the entire class was rebuilt between 1956 and 1960. A main argument was the high consumption due to the special valve gear. The casing was removed and each cylinder was equipped with its own, conventional Walschaert valve gear. Additionally, the boiler pressure has been reduced from 280 to 250 psi to simplify maintenance. The Light Pacifics were also rebuilt in the same way.

Despite the lower boiler pressure, the rebuilt machines delivered very good performances. The highest recorded speed was 105.88 mph or 170 km/h. After the South Western Main Line was handed over to Western Region administration and the class 42 diesel-hydraulics were introduced, the Merchant Navy was withdrawn between 1964 and 1967. Today, eleven of the 30 locomotives are still preserved, five of which were at least at one time operational again. These are all in rebuilt condition, but No. 35011 “General Steam Navigation” is being checked to restore it to its original condition.

After Bulleid had introduced his innovative Pacific of the Merchant Navy class, the Southern Railway identified the need for a lighter locomotive that was also suited for express and fast freight trains. On the one hand these were needed for the lighter rails in the West Country, but also for many lines which were not maintained well during the war and for which the Merchant Navy was too heavy. This included the haulage of express freights on the electrified suburban lines without slowing down the electric multiple units which operated on the same tracks.

After evaluating different wheel arrangements, Bulleid decided to build a Pacific that was a scaled-down version of the Merchant Navy, along with all its innovations. This was accomplished by the mounting of a smaller boiler, decreasing the overall length and reducing the cylinder dimensions. In the result, they were five tonnes lighter, but had the same features like the Bulleid chain-driven valve gear and the air-smoothed casing that was primarily intended to facilitate cleaning of the locomotives and also brought them the nickname “Spam Cans”. The cab was also reduced in size to fit the loading gauge of some lines and some got a smaller tender for use in the West Country.

Between 1945 and 1951, 110 locomotives were built at the SR's Brighton and Eastleigh works. The first 48 locomotives were named after places in the West Country, leading to the name “West Country class”. Since they were also intended to be used in other regions, all which followed were named after subjects or persons associated with the Battle of Britain and the RAF, hence the second name “Battle of Britain class”. They were also known by their common name “Bulleid Light Pacifics”, while their heavier sisters were called “Bulleid Heavy Pacifics”.

As with their sisters, it became evident that they were expensive to maintain, mostly due to their chain-driven valve gear. So they were rebuilt in the same way, what included the removal of the casing and the fitting of a more conventional Walschaerts valve gear. Additionally, the boiler pressure was reduced from 280 to 250 psi. Although these changes reduced their power, they reduced the maintenance costs by 60 percent and also reduced the coal consumption a bit.

The rebuilding process was halted in 1961 after 60 locomotives had been rebuilt. This was due to the fact that more and more lines in the Southern Region had been electrified, alongside the introduction of diesel locomotives on other lines. The withdrawals started in 1963 and were completed in July 1967 when all steam service ended in the Southern Region. 20 locomotives were preserved, what includes a mix of rebuilt and unrebuilt ones. Eight of them are currently operational, with more either under overhaul or awaiting the start of their overhaul.