In the late 1960s, when British Railways adopted TOPS – Total Operations Processing System, a computerised system to handle all their traffic – all existing locomotives were grouped into 'Types' according to their specified traction effort or pulling power, expressed in horsepower (hp). Within each Type, the locomotives were then sub-divided into Classes, each class representing a particular design.

Classes 21-31 were Type 2 locomotives in the power range 1,001 to 1,499hp.

The Type 2s looked more like what one would consider a classic locomotive shape, having matching cabs at each end. It can be argued that the Type 2s were a lasting hangover from the Midland Railway's influence on British Railways. The Midland was a pre-grouping company that shunned powerful locomotives for small, efficient steam engines running many small trains to a tight schedule. As loads got larger, the small locomotives had to be paired up, obviously at a loss of efficiency as two engines were being fired and crewed instead of one. The Type 2s were able to handle the smaller freight and passenger workings they were built for, but when loads were consolidated, this often meant using two locomotives where one larger one would have been good enough. The introduction of the High Speed Train in the late 1970s and early 1980s was a cause of many of the Type 2s being scrapped, as the Type 5 and Type 4 locos took over the Type 2's duties.

Class 21

The first batch of ten of these Bo-Bo¹ diesel-electric locomotives were built by the North British Locomotive Company for evaluation in 1958. They featured a twelve cylinder 1,000hp engine built by NBL under licence from the German company MAN. More orders were placed and the first 38 found service in the Eastern Region. Another 20 were built for use in Scotland with a more powerful engine; production finished in 1960.

They tipped the scales at nearly 74 tons and could hit 75 mph. They had a flat, round-topped nose with slightly raked-back windows.

The engine soon proved unreliable as it was not of the same quality as the German originals. The couplings between the engine and generator also failed. All 58 locos were moved to the Scottish region so that they could be repaired by the nearby NBL works, however that works closed in 1962. Twenty of the class were re-engined to become Class 29s, while all the others were scrapped.

The Class 21 number has been used more recently on a few French freight locomotives that have found their way onto the network through the Channel Tunnel.

Class 22

Known as the 'Baby Warships', these were diesel-hydraulic B-Bs built by NBL. They were outwardly identical to the Class 21s and bore a passing resemblance to the larger Warship express locomotive. Power was provided by the same NBL 1,000hp engine as the Class 21, but thanks to its lighter transmission, it weighed only 65 tons.

Like the other Hydraulics, they were sent to the Western Region, but suffered similar reliability problems to their Class 21 cousins. However, when NBL went bankrupt in 1962, British Rail had to use withdrawn locomotives to supply spare parts for the 'Baby Warships'. Reliability had increased thanks to the introduction of more powerful Hydraulics on the network, allowing the 'Baby Warships' to work less demanding duties.

The class was fully withdrawn by 1971. It was agreed that one would be preserved, but it was accidentally cut up by Swindon Works, a larger Warship being preserved instead.

The Class 22 designation was reused for classify a set of French electric locomotives that hauled freight through the Channel Tunnel.

Class 23

From the 'Baby Warships', to the 'Baby Deltics'. These were built by English Electric at the Vulcan Foundry using a smaller, nine cylinder version of the famous Deltic horizontally opposed engine. It gave the Bo-Bo locomotive 1,100hp and a top speed of 75mph. They used electric transmission. Ten were built in 1959.

Design-wise, they were similar to other English Electric designs, with a snout and set back raked cab. The snout contained some electrical equipment and also was thought to prevent drivers becoming hypnotised by watching the rails pass underneath them. The snout was lower than on the larger Deltic, being more like the English Electric Type 3s and 4s (Class 37s and 40s). Their snouts, like the rest of the locomotive, were also much shorter than their larger relatives.

British Rail had wanted these locomotives to come in at under 72 tons, but many of the sub-contracted components like the steam boiler came in overweight. A program of weight reduction was embarked on, but even when finished, they still had to limit the amount of sand the sanders² had on board. The Deltic engine was a way of producing a powerful and light engine, and the larger Deltic locomotive was both more powerful and lighter than other express locomotives. It seems strange that the 'Baby Deltic' suffered from weight issues

They were originally used for suburban services out of Kings Cross. Many of the duties that BR had envisioned for them were ruled out due to the weight of the locomotive, and the services into the sub-surface platforms in Moorgate were soon abandoned due to the engine's smoky nature.

The class developed a reputation for reliability issues, managing only a quarter of the mileage per failure as some of the other Type 2s. Aside from a cylinder lining issue, the engines were initially reliable. However it was the failure of ancillary equipment, especially the shaft, connected to the gearbox, that drove the cooling fans, which could snap, rupture coolant pipes and result in the engine seizing.

By 1960, there had already been 44 engine changes between the ten locomotives, with four of them out of service at any one time. They couldn't replace the engine with something more reliable because that would add many tons to the weight. They were withdrawn from Stratford Depot in 1963 before they were refurbished with new parts and a two-tone green paint scheme similar to the larger Deltics. The new-look 'Baby Deltics' were then reliable performers, however this was not to last.

The National Traction Plan of the late 1960s aimed to rationalise the locomotive fleet, and the ten non-standard 'Baby Deltics' with their mixed reputation were an easy target. They were withdrawn from service by 1971, with one running departmental test trains until 1975. None were preserved.

A project is underway to build a replica 'Baby Deltic' at Barrow Hill in Derbyshire. The engine and generator from the last 'Baby Deltic' has been restored and built into an English Electric Class 37. The locomotive is undergoing extensive reworking, including shortening the body and nose, and changing where the grills and horns are located. The Class 37's bogies have been replaced by those from a Class 20.

Class 24

These were a class of 151 Bo-Bo Diesel-Electrics built at the BR works in Derby, Darlington and Crewe from 1958 to 1961. They featured a six cylinder, 1,160hp turbo charged Sulzer engine, leading to them being known as the Sulzer Type 2s. The transmission and generators came from BTH.

They had a flattish front with lightly raked back windows. There are three windows at the front, near the top of the locomotive. The left and right windows are squarish while the central window is smaller. The locomotives were fairly heavy for their size, tipping the scales at 80 tons, with a lighter weight version at 74 tons. Although the locomotives were originally set for work around Crewe and Derby, but most of the first batch went to cover delays in electrification on the Southern Region. Due to their weight, most had their steam train-heating boilers removed.

Class 24s saw service all around the country, mainly on freight duties. They were a successful design, if limited by their lowish power and high weight. Although Class 24s were already being withdrawn through accident damage, the withdrawals started in earnest with the closure of the Waverley Line in Scotland and the completion of the Glasgow electrification project in 1974. By 1976 there were only ten Class 24s in service. The last passenger duty undertaken by a Class 24 was rescuing a Holyhead to Euston train that had failed at Colwyn Bay. Four Class 24s have been preserved.

Class 25

The Class 25s were an advance on the Class 24, based on an uprated version of the Sulzer engine. They were also known as Sulzer Type 2s, although they were commonly known as 'Rats', because the 327 strong fleet got everywhere. The locomotive had an extra 90hp, and a higher top speed of 90mph (compared with 75mph of the Class 24). They were built at various British Rail workshops between 1961 and 1967.

They were lighter than the Class 24s, thanks to weight-saving techniques in the design and construction, with weights ranging from 72 to 77 tons. Instead of the rounded roof at the front of the 24, the 25 had a headcode box³.

They were mainly for freight use, but they also saw a lot of passenger use, with a fair number of the fleet having train-heating boilers.

Three Class 25s were withdrawn from hauling trains in 1983 and converted to mobile generators to allow electric heating in trains when the hauling locomotive were not fitted with electric heating. These were known as ETHEL (Electric Train Heating Ex-Locomotives) and were not universally popular. They tended to appear between steam locomotives and coaches on excursions, and photographers and other enthusiasts thought they spoilt the lines.

They were withdrawn from service in 1987 and twenty found their way into preservation.

Class 26

The Class 26 was another Bo-Bo class of small locomotives. They looked similar to the Class 24, but their fronts were gently rounded. They were built by the Birmingham Railway Carriage and Wagon Company and so were known as the BRCW Type 2s. They also used the Sulzer 6LDA-28 engine, giving out 1,160hp, allowing the locomotive to hit 80mph. The locomotives were 50ft (15m) long and weighed between 74 and 79 tons. Forty-seven were built between 1958 and 1959.

The first batch were sent to work from the Hornsey depot in North London on the Eastern Region lines. Another second batch ended up in Scotland. After the pilot period was over, all the locomotives were split between Haymarket and Inverness in Scotland.

They were seen on both passenger and goods workings, including the Inverness to Perth leg of The Royal Highlander sleeper service to London, where they often worked in threes on the heavy train.

The class proved useful and reliable, which was an important factor in their survival when British Rail was confronted with a surplus of small locomotives. The larger Class 37s and 47s took over most of the passenger duties and some of the freight from the late 1970s, however because the Class 26's lower-powered engine was more reliable than some of the other Type 2s that used the Sulzer engine, they took over from Class 25s and 27s. The locomotives lasted in service until 1993. Thirteen remain in preservation.

Class 27

These Bo-Bo locomotives were built between 1961 and 1962 by BRCW and were a development of the Class 26. Like their older cousins, they were referred to as BRCW Type 2s. They featured the same engine, but rated at 1,250hp, giving it a top speed of 90mph. They were quite a few tons lighter than the Class 26s.

Some of the 69 locomotives were allocated to Scotland, the rest to Leicester and Cricklewood in London. The Cricklewood locomotives were used for freight services across London and also boat trains to Tilbury. By 1969 the entire class was moved to Scotland, allowing them to replace the Class 17s. They saw a lot of service on the West Highland Line.

Class 27s were brought in to replace multiple units on the Edinburgh to Glasgow Queen Street express services. Pairs of locomotives were attached to each end of a six-coach set. Both of the locomotives would be fitted with special equipment to allow them to work the trains, which also had air brakes rather than the usual vacuum system. On one end the locomotive also had electric train-heating; these were classified Class 27/2s, the ones on the other end were Class 27/1s. Running services at 90mph took their toll on the locomotives, decreasing reliability, especially in the 27/2s. In 1980 these services were taken over by a single Class 47 locomotive.

Due to the class being less reliable than the older Class 26, they were removed from service by 1987 with eight remaining in preservation.

Class 28

While the pilot program threw up many unusual and often unsuccessful designs, it was a comment on how unconventional the lone-surviving Class 28 was that it acquired the nickname 'The Object'.

Twenty of this class were built by Metropolitan-Vickers to evaluate how a low-revving two-stroke engine would work in a locomotive. While the Crossley HST V8 engine gave the class a huge tractive effort for a Type 2 locomotive, it was also very heavy, with the Class 28 tipping the scales at 97 tons. This meant that it had to have an extra axle, leaving it with a Co-Bo wheel arrangement, unique on British Rail. Having the extra axle meant that it didn't have any issues with wheelslip, but the uneven weight distribution led to restrictions as to where it could work and caused problems with maintenance. They were built between 1958 and 1959 and were known at the Metrovicks.

The locomotives were 56ft (17m) long with flat fronts with three windows in them. Originally the windows were wrap around, which gave the driver improved visibility and were a rare aesthetic feature on an ungainly loco.

They were sent to the Hendon depot for use on the Condor Expresses. These were high-speed freight services that ran non-stop from London to Carlisle for a crew change and then on to Glasgow. The trip was the longest locomotive run of its day and the non-stop section was longer than any other passenger or freight service.

The Condor was introduced in response to the oil crisis which saw freight traffic move back from road haulage to the railways. However, it was introduced two years too late when freight was already returning to the road network. The trains consisted of 24 wagons with two containers on each and called for two Class 28s to haul it. They were rostered in pairs and if one of them was faulty, the pair were replaced by Stanier Black 5 steam locos. A failure on route would see just the faulty loco replaced by a Black 5, which were easy enough to find along the route.

For the locomotives that found themselves stuck in Scotland without a return working, they were given a lot of hard work, until reliability issues caused the bosses to make sure that the locomotives were fit for their return to London.

The locomotives also worked services into Moorgate from Bedford, and the Manchester to St Pancras expresses. On the latter, they often worked with Stanier steam locomotives.

The engines were the major issue with the class. The Crossely engines, when running correctly, were very smoky as they burnt oil. When not running correctly, they were a maintenance nightmare. Fireballs were reported to be fired though the exhaust and the engine was even claimed to have shaken itself off its mounting. This was very strange as the Crossely engine revved more slowly than any other locomotive engine⁴.

The other issue with the locomotive was cosmetic, the windows kept falling out. Fixes were introduced which solved the window problem but didn't bring a solution to the engine issues. The locomotives' workings were fairly difficult, often taking on many of the difficult climbs of the network. At one point only three of the class were available for service.

There was a plan for the class to have their engines replaced, allowing them to be useful engines, but it was decided that since they were only a small class of non-standard locomotives, their time in service was over by 1968, before many of the steam locomotives they were due to replace were retired.

One locomotive was put to use in the Research Division, then was used for carriage heating before being left in a siding. Eventually it found its way into preservation.

The character BoCo from the Thomas the Tank Engine stories was a Class 28.

Class 29

The Class 29s were twenty Class 21 locomotives re-engineered to use the fast-revving Paxman Ventura egine. Power was boosted to 1,350hp and reliability was much improved. This didn't count for much as the class was small and non-standard and so were taken out of service by 1972. None survived into preservation.

Class 30 and 31

These 263 locomotives originally started life under BR's Pilot Scheme⁵. They were built by Brush Traction using 1,250bhp and 1,365bhp Mirrlees JVS12T engines and were known as the Brush Type 2s. The building started in 1957 and concluded in 1962. They were one of the larger Type 2s, being upwards of 106 tons⁶. Because of their weight, they had to have an extra axle at each end to support them. This meant that they were the only A1A-A1A locomotives on the network.

Despite Mirrlees proudly advertising the use of their engine in this new fleet of locomotives, the locomotives proved unreliable in service and were replaced by more powerful (1,470bhp) English Electric 12SVT power units in 1964. The result of this change was the reclassification from Class 30 to Class 31.

The locomotives were 56ft (17m) long and looked similar to the Class 24 and 25 with three front windows, the middle window not being as tall. The first batch of 20 (Class 31/0) did not have a large headcode box on the roof of the cab but the rest did. This first batch was limited to 80mph and had non-standard control equipment for multiple working. The later locomotives were rated for 90mph.

The 31/4 subclass were 69 locomotives fitted with electric train-heating. They were able to heat eleven carriages, allowing them to heat long rakes of coaches as they took them from sidings to stations for more powerful express locomotives to haul. Their own trains were much shorter. A side effect of the train heating was that the equipment took a third of the locomotive's power. Even when the equipment was turned off, the locomotive couldn't use its full power. Given that the class had a lower tractive effort than many other Type 2s, it meant that this heavy locomotive was even more underpowered. They were often at the head of late-running trains.

The class was mainly used on the East Anglian lines, although they also were used on the rest of the Eastern Region and appeared in the Midland and Western Regions. Despite the other Type 2s being both lighter and more powerful, the Class 31s were chosen to become the standard Type 2, mainly on accounts of their reliability. Many of the Class 31s survived British Rail and found their way into the hands of both commercial operators and Railtrack. Over 30 have survived into preservation.