||The examples and perspective in this article may not represent a worldwide view of the subject. (December 2010)|
A track pan (American terminology) or water trough (British terminology) is a device to enable a steam railway locomotive to replenish its water supply while in motion. It consists of a long trough filled with water, lying along a flat stretch of railroad/railway track between the rails.
When a steam locomotive passes at speed over the trough, its water scoop can be lowered, and the speed of forward motion forces water into the scoop, up the scoop pipe and into the tanks or locomotive tender.
Track equipment 
Main line steam locomotives consume considerable volumes of water, and the tender or side tanks need to be replenished at intervals. Where long distance non-stop running is contemplated, there is a need to replenish without stopping.
In 1860 John Ramsbottom, a Victorian locomotive engineer, devised the water trough/track pan to meet this need. It is a very long (several hundred yards) open-ended shallow trough laid between the rails. A nearby water source is needed to recharge the trough as passing trains take water. The entire length of the installation must be on level track, with a short section of rising track at each end to ensure that the water is retained. As trains needed to be moving at some speed to pick up enough water, a suitable separation between trough locations and commercial stopping points is essential.
The first installation was brought into use on 23 June 1860 at Mochdre, Conwy, on the North Wales main line of the London and North Western Railway (LNWR) about mid way between Chester and Holyhead.
The siting of the troughs requires a long enough length of straight and level track (although very large radius curves could be accommodated). There must be a good water supply nearby. In hard water areas, water softening plant may have been considered necessary.
Locomotive equipment 
A scoop is fitted to the underside of the locomotive's tender (or the locomotive itself in the case of tank locomotives) in such a way that it can be raised or lowered, by a hand operated screw or a power mechanism. The scoop feeds into a vertical pipe that discharges into the water tank.
The scoop needs to be lowered at speed at the correct location - shortly after the start of the trough - and raised again when either the tank is full, or at the end of the trough. Lineside indicators are provided to assist engine crews in determining the location; in the UK it was a large white rectangular board with a black horizontal zigzag marking. On American railroads, illuminated trackside signals were employed for night-time usage, to indicate the start and approaching end of the track pan.
Venting on the tender needed to be free to allow a high rate of release of expelled air from the tank. Tank locomotives were generally fitted for picking up in either direction, but tender locomotives generally picked up in the forward direction only.
Operational considerations 
The LNWR quickly installed water troughs at other locations, but other companies were slow to adopt the new apparatus. The Great Western Railway did so from 1895, and subsequently all the major railways in Great Britain, with the exception of the lines south of the River Thames, installed the equipment. Taking water at speed results in considerable spray behind the scoop; this risks drenching passengers in the leading vehicles, and in Great Britain it was customary for the guard or other traincrew to warn passengers in the first coach to keep the windows closed. In one incident on the LMS railway in Britain, two streamlined trains with "Coronation" class locomotives happened to pass each other at a water trough when one of the trains was taking on water. The other train suffered broken windows due to lumps of tender coal scattered by the spray and the complaints from drenched passengers caused the management to retimetable the trains to ensure this could not happen again. Vaughan says that the Royal Train when conveying royalty was not permitted to be passed by another train in a section where there was a water trough.
Vaughan states that the GWR investigated the effectiveness of varying train speed, and found that 45 mph was the optimum speed; but water could be picked up successfully as low as 15 mph. At that speed 944 gallons could be picked up in 440 yards, but Vaughan suggests that this is a low theoretical figure, and that it overlooks the bow wave effect which enables a greater take-up rate. There was a significant resistance to the forward motion of the engine during the process, enough to require special care by the driver to avoid problems on unfitted freight trains.
The considerable water spray made track maintenance difficult, and the physical trough equipment limited access for packing sleepers, exacerbating the problem. In very cold weather the water would freeze, preventing water pick-up, unless a heating apparatus was installed.
Track pans normally took a while to fill up after being used, so they could not be used immediately by a close-following train. They were also expensive to maintain, generally requiring a pumping station, a lot of plumbing, and an employee or two to maintain. They were thus only justified on a railroad with a high traffic volume. In the United States, several big eastern railroads used them, primarily the New York Central and Pennsylvania Railroad.
Use by diesel locomotives 
Diesel locomotives were introduced in the United Kingdom by British Railways in the 1950s, working alongside steam traction until 1968. Passenger vehicles were heated by steam from the locomotive boiler at that time, and the early diesel locomotives were provided with auxiliary boilers to provide the steam. Locomotives intended for long non-stop runs (such as the Class 40 and Class 55) were fitted with water scoops to allow them to replenish the steam generator's water supply from troughs/pans. The withdrawal of steam traction and the introduction of rolling stock with electric rather than steam heating removed the need for such equipment on later types and scoop-equipped locomotives had their scoops removed.
A map showing the location of GWR troughs in the 1930s is reproduced in the book 'The Great Western Railway'. They are typically at spacings of 40 to 50 miles, but with some wide variations. There are a few instances of trough locations very close to major stopping points; for example Fox's Wood, near St Annes Park, two miles from Bristol Temple Meads; however this was installed when trains to South Wales travelled via Bath and Filton, using these troughs; after the opening of the South Wales direct route via Badminton, numerous passenger and goods trains continued to use the route and required the troughs. The lengths are also given: they vary from 524 to 620 yards (480 to 570 metres).
The locations were (in 1936):
|Location||Mileage||In use from||Length (yards)|
|Pangbourne - Goring||43½||1 October 1895||620|
|Aldermaston - Midgham||45½||by 1904||620|
|Fairwood Junction (up)||111½||-||553|
|Fairwood Junction (down)||111¾||-||495|
|Cogload Jn - Creech Jn||159¼||March 1902||560|
|Exminster - Starcross||200||July 1904||560|
|Keynsham - Fox's Wood||114¾||1 October 1895||620|
|Chipping Sodbury||104||1 January 1903||524|
|Undy - Magor||150¼||-||560|
|Denham - Ruislip||2¼||20 November 1905||560|
|Rowington Jn||114½||by July 1902||440 (560 from 1908)|
|Bromfield - Ludlow||22½||-||613|
The practice of drafting water in this way gave rise to the American expression, "Jerkwater Town," used generally to indicate a small town or out-of-the-way place. If a town were sizable, it would have a water tank for which the train would stop to take on water. If the town didn't rate a water tank, it would be forced to "jerk" water while moving to refill the engine's tender[dubious ].
Alternative techniques 
Railway companies were well aware of the cost of installing and maintaining this equipment, and the provision of tenders with a large water capacity was an alternative employed in some cases.
See also 
- Acworth, J M, The Railways of England, 1889, John Murray, London
- Vaughan, Adrian (1990). "Water troughs on the GWR". Railway World 51: 278–80, 370–4.
- Robbins, Michael (1967). Points and Signals. London: George Allen & Unwin.
- Foster, Richard (1989). "L&NWR water troughs". British Railway Journal (London & Birmingham Railway edition): 84–91.
- Twells, H. N. (1982). LMS Miscellany: a pictorial record. Oxford: Oxford Publishing Co. ISBN 0-86093-172-2.
- "Water pick-up troughs". Railway Magazine 74: 4–7. 1934.
- Webb, David (1984-8). "Water troughs". Cumbrian Railways Circular 3: 223, 263–4.
- The Great Western Railway - 150 Glorious Years, Whitehouse P and Thomas St John (editors), David & Charles, Newton Abbot, ISBN 0-7153-8763-4
- Great Western Railway, Generla Appendix to the Rule Book, 1936, quoted by Vaughan; the Lostwithiel entry comes from H Holcroft, An Outline of Great Western Locomotive Practice, quoted by Vaughan
- Feldman, David. Imponderables
-  Vintage cine of British steam engines in the 1950s using water troughs/track pans