A '''tank locomotive''' is 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'' and others.

thumb|right|216px|A side tank
===Side 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 LNER by Gresley and on the Deeley. 
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thumb|right|216px|A saddle tank

===Saddle tank===<!-- please do not rename this section as it is being used as a target by another article -->
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. 
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thumb|right|216px|A pannier tank

===Pannier tank===<!-- This section is linked from River Severn -->
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<ref>[http://myweb.tiscali.co.uk/kevin.dare/general%20info.html general info<!-- Bot generated title -->]</ref> 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<ref>Holcroft, H: "An outline of Great Western locomotive practice 1837-1947" Locomotive Publishing Company, London, U.K. (undated)</ref>. 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. <ref>Dambly, Phil: "Nos inoubliables 'Vapeur'. Editions LE RAIL, Brussels (1968)</ref> 
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thumb|right|216px|A well tank

===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.
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thumb|250px|A rear 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 loco.
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=== 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.  

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===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) as well.  This is both to increase the water capacity and to equalise the weight distribution.

thumb|right|300px|South African Railways  gauge NGG16 class Garratt, preserved in operating condition in Wales.

===Garratt locomotive===

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 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.
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right|thumb|200px|Танк-паровоз серии 9П.
'''Танк-парово́з''' — тендера. Запас воды и топлива хранится на самом паровозе. Танк-паровозы имели ограниченное применение в местах, где от них требовалась большая подвижность и были постоянно доступны топливо и вода. Например, в СССР по схеме танк-паровозов строились маневровые паровозы серии 9П. В качестве модификации 9П также изготавливались т. н. «бестопочные» паровозы, предназначенные для работы на пожароопасных производствах. Такой паровоз заправлялся паром от внешнего источника и дальше работал некоторое время до падения давления в котле.

thumb|right|200px|Бестопочный паровоз (заправка производится через кран спереди).