Tank locomotive
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A tank locomotive (also tank engine in the United Kingdom) is a steam locomotive that carries its own fuel and water on it, instead of pulling it behind it in a tender.
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[edit] Types of tank
There are a number of types of tank 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.
[edit] Side tank
The water is contained in rectangular tanks mounted on each side of the locomotive, next to the boiler. This is the most common configuration. The side tanks can restrict access to the valve gear.
[edit] Saddle tank
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 especially 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, but the locomotive has a higher centre of gravity and hence must operate at lower speeds.
[edit] Pannier tank
This variety of tank, used almost exclusively by the British Great Western Railway, and in Belgium, involved box-shaped tanks attached to 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 footplate. This lowered the centre of gravity compared to a saddle tank, whilst allowing access to the valve gear.
[edit] Well tank
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.
[edit] 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.
[edit] 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.
[edit] 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.
[edit] 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 tank locomotives 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.
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.
[edit] Popularity
Worldwide, tank locomotives 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.
In Australia, the main engines used on the rebuilt West Coast Wilderness Railway in Tasmania are reconditioned tank locomotives.
[edit] Preservation
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.
