Tank locomotive explained

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 (or fuel tanks) 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.^[1] 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 which 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.^[2] 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 (inside motion). 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'.

Saddle tank

The water tank sits on top of the boiler like a saddle sits atop a horse.^[3] 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).^[4] Walter Nielson patented the saddle tank arrangement in 1849.^[5]

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.

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.^[6]

Britain

In Britain, pannier tank locomotives were used almost exclusively by the Great Western Railway. The first Great Western pannier tanks were converted from saddle tank locomotives^[7] 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.^[8] Pannier tank locomotives are often seen as an icon of the GWR.

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 (SGE), a private company grouping smaller secondary lines.^[9]

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.^[10] 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.^[11] 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.

Wing tank

Wing tanks are side tanks that run the length of the smokebox, instead of the full length of the boiler.^[12] In the early 19th century the term "wing tank" was sometimes used as a synonym for side tank.^[13]

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.^[14]

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.^[15] 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.^[16]

Tender-tank

A tank locomotive may also haul a tender behind it.^[17] 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 (where coal was used) of 1 pound of coal for every 6 pounds 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.

Combinations

Large side tank engines might also have an additional rear tank (under the coal bunker), or a well tank (between the frames).^[18] This may have been to increase the water capacity, to equalise the weight distribution, or else improve the stability by lowering the centre of gravity.^[19]

Locomotive classification and wheel arrangement

Because tank locomotives are capable of running equally fast in both directions (see below) they usually have symmetrical wheel arrangements to ensure the same ride and stability characteristics regardless of the direction travelled, producing arrangements with only driving wheels (e.g. and) or equal numbers of leading and trailing wheels (e.g. and).^[20] However other requirements, such as the need to support a large bunker, would require a non-symmetrical layout such as .

Whyte classification

In the Whyte notation for classification of locomotives (primarily by wheel arrangement), various suffixes are used to denote tank locomotives:

Notes and References

1. Book: Steam Locomotives . P. M.. Kalla-Bishop . Luciano . Greggio . Crescent Books . 1985.
2. Book: Locomotive Cyclopedia of American Practice . 1922 . Simmons-Boardman Publishing Corporation . 86.
3. Book: Joseph Gregory Horner . Lockwood's Dictionary of Terms Used in the Practice of Mechanical Engineering . 1892 . Crosby, Lockwood and Son . 413.
4. Book: Gordon Edgar . Industrial Locomotives & Railways of Scotland . 15 October 2019 . Amberley Publishing . 978-1-4456-4943-6. 220.
5. Book: George Augustus Nokes . The Evolution of the Steam Locomotive (1803 to 1898) . 1899 . Railway Publishing Company . 11.
6. Book: Robin Jones . Great Western Railway Pannier Tanks . 31 January 2014 . Crowood . 978-1-84797-654-3.
7. Web site: Pannier tanks . https://web.archive.org/web/20160514015545/http://myweb.tiscali.co.uk/kevin.dare/general%20info.html . 2016-05-14.
8. [Harold Holcroft|Holcroft, H]
9. Dambly, Phil: "Nos inoubliables 'Vapeur'. Editions LE RAIL, Brussels (1968)
10. News: An old "well" tank locomotive . Locomotive, Railway Carriage and Wagon Review . 1908 . Locomotive Publishing Company . 218.
11. News: Tank locomotives for suburban service on American railways . Engineering News . 1905 . McGraw-Hill Publishing Company . 168.
12. Book: Nehemiah Hawkins . Hawkins' Mechanical Dictionary: A Cyclopedia of Words, Terms, Phrases and Data Used in the Mechanic Arts, Trades and Sciences . 1909 . T. Audel . 655.
13. Book: Institution of Mechanical Engineers (Great Britain) . Proceedings of the Institution of Mechanical Engineers . 1864 . The Institution . 103.
14. Book: Cozens, Lewis . The Van and Kerry Railways: With the Kerry Tramway . 1953 . R. Cozens.
15. Book: Mark Smithers . The Royal Arsenal Railways: The Rise and Fall of a Military Railway Network . 31 March 2016 . Pen & Sword Books . 978-1-4738-4401-8 . 192.
16. Engineering . Narrow Gauge Locomotive For the Gas Light and Coke Company . 7 June 1895 .
17. Book: Anthony Burton . John Scott-Morgan . The Light Railways of Britain and Ireland . 30 November 2015 . Pen and Sword . 978-1-4738-2706-6 . 18–19.
18. Book: Lowe, James W. . British Steam Locomotive Builders . 2014 . Pen & Sword Books Limited . 978-1-4738-2289-4.
19. News: Locomotives built between 1930 and 1962 . Trains . 1964 . Kalmbach Publishing Company . 24. 35.
20. Book: Joseph Russell Howden . The Boys' Book of Locomotives . 1909 . F. A. Stokes Company . 177–179.