Tank locomotive

A tank locomotive is a steam locomotive that carries its own fuel and water on it, instead of pulling it behind it in a tender.

Origins

The earliest tank locomotives date from the 1840s, one of the first of which was supplied by George England and Co. of New Cross to the contractors building the Newhaven, Sussex branch line for the London Brighton and South Coast Railway in 1848.^[1] In spite of the early belief that such locomotives were inherently unsafe,^[2] the idea quickly caught on, particularly for industrial use and five manufacturers exhibited designs at The Great Exhibition in 1851. These were E. B. Wilson and Company, William Fairbairn & Sons, George England, Kitson Thompson and Hewitson and William Bridges Adams.^[3]

Types of locomotive

There are a number of types of locomotive, based on the location and style of the water tanks. These include the side tank, the saddle tank, the pannier tank, the well tank and others.

Side tank

A side tank

An example with a tapered front and an access aperture

A configuration very common in the U.K. The water is contained in rectangular tanks mounted on either side of the locomotive, next to the boiler. The tank sides usually extend down to the running platform, if such is present. The length of side tanks was often limited in order not to restrict access to inside motion. If it was desired to extend them to the front of the locomotive for greater capacity, access could be facilitated by apertures provided at the appropriate location. With larger side tanks it was sometimes necessary to taper the tanks slightly at the front end to improve forward visibility.

Saddle tank

A round saddletank

A saddle tank with straight sides

The water tank sits on top of the boiler like a saddle. Usually the tank is curved in cross-section, although in some cases there were straight sides surmounted by a curve (like an inverted 'U') , or even an ogee shape (a concave arc flowing into a convex arc). Saddle tanks were a popular arrangement especially for smaller locomotives in industrial use. It gave a greater water supply, but limited the size of the boiler and restricted access to it for cleaning. The water is pre-heated by the boiler, which reduces the loss of steam found when cold water is injected into the boiler. However, the locomotive has a higher centre of gravity and hence must operate at lower speeds. Also, if used with hot water, injectors lose efficiency and can fail.

Pannier tank

A pannier tank

Pannier tanks, used almost exclusively in Britain by Great Western Railway, and common in Belgium, involved box-shaped tanks carried on the sides of the boiler. Unlike the side tank, they did not go all the way down and there was space between the tank and the running plate. The pannier arrangement lowered the centre of gravity compared to a saddle tank, whilst still allowing the easy access to the inside motion that the latter gave. The first GWR Great Western pannier tanks were actually converted from saddle tank locomotives^[4] when these were being rebuilt in the early 1900s with the Belpaire firebox. There were difficulties in accommodating the flat top of the latter within an encircling saddle tank which cut down capacity and increased the tendency to overheat the water in the tank^[5]. In Belgium, pannier tanks had been in use at least since 1866, once again in conjunction with Belpaire firebox locomotives built for the Belgian State and for la Société Générale d'Exploitatation (SGE), a private company grouping smaller secondary lines. ^[6]

Well tank

A well tank

A well tank formerly used on London suburban services

In this design, used in earlier and smaller locomotives, the water is stored in a 'well' on the underside of the locomotive, generally between the locomotive's frames. This does not restrict access to the boiler, but space is limited there, and the design is therefore not suitable for locomotives that need a good usable range before refilling. The arrangement does, however, have the advantage of creating a low centre of gravity, creating greater stability on poorly laid or narrow gauge tracks.

Rear Tank (or Back tank)

A rear tank.

In this design, the tank is placed behind the cab, usually over a supporting bogie. This removes the weight of the water from the driving wheels, giving the locomotive a constant tractive weight. The disadvantage is a reduction in water carrying capacity. A rear tank is an essential component of the American Forney type of loco.

Wing Tank

Wing Tanks are side tanks that run the length of the smokebox, instead of the full length of the boiler. They were mainly used on narrow gauge industrial locomotives that could be frequently re-filled with water and where side or saddle tanks would restrict access to inside valve gear. See Kerry Tramway Excelsior which has been described, by various sources, as both a wing tank and an inverted saddle tank.

Inverted Saddle Tank

The inverted saddle tank was a variation of the Wing Tank where the two tanks were joined underneath the smokebox and supported it. This rare design was used for the same reasons as the wing tank but provided slightly greater water capacity. The Brill Tramway locomotive Wotton is believed to have had an inverted saddle tank. The inverted saddle tank seems to have been a speciality of W.G.Bagnall.

Combinations

Large side tank engines often have a rear tank (under the coal bunker), or a well tank (between the frames) as well. This is to increase the water capacity, to equalise the weight distribution, and improve stability.

Fuel bunker

A rear bunker.

A side bunker.

On a tank locomotive the fuel (most often coal) is carried in a bunker the location of which can vary. On a locomotive with a trailing carrying axle or a trailing bogie the bunker is generally situated to the rear of the cab, but in cases where the firebox overhangs the rear driving axle, it has been common practice to situate the bunker on top of and to one side of the firebox; this concentrates the weight and stabilises the locomotive.

Other types of tank locomotive

There are several other specialised types of steam locomotive which carry their own fuel but which are usually categorised for different reasons.

Garratt locomotive

South African Railways NGG16 class Garratt, preserved in Wales.

Main article Garratt

A Garratt type of locomotive is articulated in three parts. The boiler is mounted on the centre frame, and two steam engines are mounted on separate frames, one on each end of the boiler. Articulation is used so larger locomotives can go round curves which would otherwise restrict the size of rigid framed locomotives. One of the major advantages of the Garratt form of articulation is the maintenance of the locomotive's centre-of-gravity over or inside the track centre-line when rounding curves. Some other forms of articulation, notably the Mallet, tend to move the centre-of-gravity outside the centre-line on tight curves, leading to problems with traction and stability.

Crane tank

A crane tank preserved as a static exhibit at Bressingham.

Main article Crane tank

A Crane tank (CT) is a steam tank locomotive with a crane fitted to it, thereby creating a small mobile crane for working in railway workshops or other industrial environments. The crane may be fitted at the front, centre or rear of the locomotive and gives it a rather "top heavy" appearance.

Fireless locomotive

A typical British fireless locomotive of 1929, built by W. G. Bagnall (works no 2370); here awaiting restoration, in Leicestershire

Fireless locomotives are tank locomotives insofar as they carry their "fuel" (steam) on board in a reservoir, known as a steam accumulator. The reservoir is filled from an external source of steam. Some types of fireless locomotives are "fuelled" by pressurized air, rather than steam.

Pros and cons

600 mm (1 ft 11+⁵⁄₈ in) gauge tank locomotive Tx26-423 in Poznań, Poland

The benefits of tank locomotives include:

• Bi-directionality: most tank locomotives are capable of running at full speed in either direction. Most tender locomotives are unable to do this, because the heavy tender is not designed to be pushed and may become unstable at higher speeds. Tender locomotives generally require turning facilities, such as a turntable or wye, at each end of the run. A tank locomotive, on the other hand, can simply run around the train and pull it back in the other direction. The crew of a tank engine generally have a better view in the reverse direction than for a tender engine and are protected from the weather.

• Fuel and water add to adhesive weight: the usable tractive weight of a locomotive is the product of the weight on its drivers multiplied by the factor of adhesion. Therefore, up to the limits of the maximum permissible axle loading, and other loading limits, the more weight on the driving wheels the better. In a tank locomotive the weight of its own fuel and water increase the available tractive weight.

• Compactness: A tank locomotive is shorter than the equivalent tender locomotive. This is important in environments limited space for locomotives, for example the headshunt of a run-round loop.

• Efficiency: Many train tanks are designed to be in contact with and be heated by, the boiler. Pre-heated water will reach boiling point faster than the colder water available from a tender. However, excessively hot water can interfere with steam injector operation and is to be avoided.

There are corresponding disadvantages:

• Limited fuel and water capacity: a tender can typically contain far more of both than is available on a tank locomotive. This restricts the range of tank locomotives between fueling and watering points.

• Varying adhesive weight: Use of the fuel and water for the purposes of adhesive weight means that the adhesive weight of the locomotive decreases as they are used up.

• Instability: Water surging inside large side tanks can cause the locomotive to become unstable and prone to derailment, as was the case with the LB&SCR L class 4-6-4T before they were modified.

• Axle loading limits a problem: For larger tank locomotives, it is hard to put much fuel and water aboard without requiring more axles than a rigid frame can handle.

Popularity

Worldwide, tank engines varied in popularity. They were more common in areas where space was at a premium, mostly Europe and other areas where the railways came later and had to fit into the towns rather than the towns growing around the rails. With their limited fuel and water capacity, they were not favored in areas where long runs between stops were the norm.

They were very common in the United Kingdom, France, and in particular Germany where examples of large tank locomotives were built. In the United States they tended to be restricted to push-pull suburban service, always the tank engine's forte, and also for switching service in terminals and locomotive shops. They were also popular in logging, mining and industrial service.

Preservation

Polish-built side tank locomotive 7646 "Northampton" at its naming ceremony in 2001

Tank locomotives are popular with heritage railways for a number of reasons. They are usually cheaper to purchase than a tender locomotive due to their smaller size. This is also an advantage when working out the cost of transportation to the heritage line, many of which are isolated from the national rail network.

Most heritage railways only have short lines and do not often have turntables at both ends of the line. A tank locomotive has good visibility for the driver in both directions. As the trains being hauled are usually light the tank locomotive is more fuel efficient than a large tender locomotive.

Add to this the lower cost of maintaining the engine and the lower amount of wear and tear on the track. Another fact to consider is that many tank locomotives were bought from former industrial railways rather than the national network. Due to the higher availability it would be easier to secure a suitable locomotive for a reasonable price.

See also

• Steam locomotive nomenclature
• Crane tank

Notes and references