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
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
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. Sometimes, when it was desired to extend them to the front of the locomotive for maximum capacity, access could be facilitated by apertures provided at the appropriate location, an arrangement used on the GNR
and on the Midland Railway
The water tank sits on top of the boiler like a saddle. Generally the tank is curved in cross-section, like an inverted 'U'. This was a fairly popular arrangement especially for smaller locomotives, and particularly in industrial use. It gave a greater water supply, but restricted the maximum 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.
This variety, 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 Great Western pannier tanks were actually converted from saddle tank locomotives 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. 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.
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.
Rear Tank (or Back 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 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
is believed to have had an inverted saddle tank. The inverted saddle tank seems to have been a speciality of W.G.Bagnall
Large side tank engines often have a rear tank (under the coal bunker) as well. This is both to increase the water capacity and to equalise the weight distribution.
The Garratt type of locomotive is a tank locomotive, in that the water and fuel supplies are carried on the engine units.
A Garratt is a type of steam locomotive that is articulated in three parts. Its 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 Mallett, tend to move the centre-of-gravity outside the centre-line on tight curves, leading to problems with traction and stability.
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.
Pros and cons
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 driver of a tank engine generally has a better view in the reverse direction than for a tender engine.
- 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.
- 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.
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.
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.