Tank locomotive
A tank locomotive is a steam locomotive which carries its water in one or more on-board water tanks, instead of a more traditional tender. Most tank engines also have bunkers to hold fuel; in a [|tender-tank] locomotive a tender holds some or all of the fuel, and may hold some water also.
There are several different types of tank locomotive, distinguished by the position and style of the water tanks and fuel bunkers. The most common type has tanks mounted either side of the boiler. This type originated about 1840 and quickly became popular for industrial tasks, and later for shunting and shorter-distance main line duties.
Tank locomotives have [|advantages and disadvantages] compared to traditional locomotives that required a separate tender to carry needed water and fuel.
History
Origins
The first tank locomotive was the Novelty that ran at the Rainhill Trials in 1829. It was an example of a well tank. However, the more common form of side tank date from the 1840s; one of the first of these was supplied by George England and Co. of New Cross to the contractors building the Seaford branch line for the London Brighton and South Coast Railway in 1848. In spite of the early belief that such locomotives were inherently unsafe, 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. By the mid-1850s tank locomotives were to be found performing a variety of main line and industrial roles, particularly those involving shorter journeys or frequent changes in direction.Types
There are a number of types of tank locomotive, based on the location and style of the water tanks.Side tank
Side tanks are cuboid-shaped tanks that are situated on both sides of the boiler, extending all or part of the boiler's length. The tank sides extend down to the running platform, if such is present, for at least part of their length. This was a common configuration in the UK.The length of side tanks was often limited in order to give access to the valve gear. Tanks that ran the full length of the boiler provided greater water capacity and, in this case, cut-outs in the rectangular tank gave access to the valve gear. Longer side tanks were sometimes tapered downwards at the front to improve forward visibility. Side tanks almost all stopped at, or before, the end of the boiler barrel, with the smokebox protruding ahead. A few designs did reach to the front of the smokebox and these were termed 'flatirons'.
Skirt tank
Skirt tanks are a rare variant of side tanks that are set below footplate level instead, for stability. They were used on some Dutch and Belgian narrow gauge tram locomotives, including large Garratt articulated types. The tanks were fitted to the power bogies, obscuring the wheels and motion. As tram locomotives had their motion enclosed anyway, the loss of access was unimportant. Dutch influence extended to the large SS 800 2-12-2 Javanic gauge tank locomotives built for the Dutch East Indies, where they were set low for stability.Saddle tank
The water tank sits on top of the boiler like a saddle sits atop a horse. Usually, the tank is curved in cross-section, although in some cases there were straight sides surmounted by a curve, or even an ogee shape. Walter Nielson patented the [|saddle tank] arrangement in 1849.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. Furthermore, the locomotive has a higher centre of gravity and hence must operate at lower speeds. The driver's vision may also be restricted, again restricting the safe speed.
The squared-off shape of the Belpaire firebox does not fit easily beneath a saddle tank, and so most saddle tanks retained the older round-topped boiler instead. A few American locomotives used saddle tanks that only covered the boiler barrel, forward of the firebox.
Water in the tank is slightly pre-heated by the boiler, which reduces the loss of pressure found when cold feedwater is injected into the boiler. However, if the water becomes too hot, injectors lose efficiency and can fail. For this reason, the tanks often stopped short of the hotter and uninsulated smokebox.
Box tank
Box tank locomotives have saddle tanks but the tank itself is box shaped. These locomotives were used globally, the most extensive user of such locomotives was the United States with many box tanks being used on the Pennsylvania Railroad on older steam locomotives built during the 19th century.Pannier tank
Pannier tanks are box-shaped tanks carried on the sides of the boiler, not carried on the locomotive's running plates. This leaves a space between the tanks and the running plate. Pannier tanks have a lower centre of gravity than a saddle tank, whilst still giving the same easy access to the valve gear. Pannier tanks are so-named because the tanks are in a similar position to the panniers on a pack animal.Belgium
In Belgium, pannier tanks were in use at least since 1866, once again in conjunction with Belpaire firebox. Locomotives were built for the Belgian State and for la Société Générale d'Exploitatation , a private company grouping smaller secondary lines.United Kingdom
In the United Kingdom, pannier tank locomotives were used almost exclusively by the Great Western Railway. The first Great Western pannier tanks were 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. Pannier tank locomotives are often seen as an icon of the GWR.United States
In Logging railroads in the Western USA used 2-6-6-2 Saddle tanks or Pannier tanks for heavy timber trains.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 arrangement was patented by S.D. Davison in 1852. 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. The first tank locomotive, Novelty, was a well tank.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 locomotive, which is a 4-4-0 American-type with wheels reversed.Back tanks were a common feature in combination with side tanks for large tank locomotives. Their coal bunker was raised above the footplate and given a sloped floor, to assist with shovelling coal from it. The space beneath, which might include also a well tank below the footplate, was then used as additional water tank space.
Wing tank
Wing tanks are side tanks that run the length of the smokebox, instead of the full length of the boiler. In the early 19th century the term "wing tank" was sometimes used as a synonym for side tank.Wing tanks 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 valve gear. The Kerry Tramway's locomotive Excelsior 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 was a speciality of W.G.Bagnall.Tender-tank
A tank locomotive may also haul a tender behind it. This was the common arrangement on the largest locomotives, as well as on narrow-gauge railways where the small size of the locomotive restricts the space available for fuel and water. These combined both fuel and water in a proportion of one unit mass of coal for every six of water..Where a tender was used with a narrow-gauge locomotive it usually carried only fuel, with water carried in the locomotive's tanks. The tender offered greater fuel capacity than a bunker on the locomotive and often the water capacity could be increased by converting redundant bunker space into a water tank.