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.
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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.
John Ramsbottom, a Victorian locomotive engineer, devised the water trough/track pan to meet this need.[1] 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 keep the water in. 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 about October 1860 by the London and North Western Railway at Mochdre, Conwy on its North Wales Coast (or Chester and Holyhead) line.[2]
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 screw mechanism or a compressed air 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 before 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 release of expelled air from the tank.
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.
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.
In Britain, they could be found on all main lines, except on the Southern Railway.[3][4][5][6][7]
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'.[8] They are typically at spacings of 40 to 50 miles, but with some wide variations, and a few instances of trough locations very close to major stopping points (e.g. St Annes Park, two miles from Bristol Temple Meads). The lengths are also given: they vary from 524 to 620 yards (480 to 570 metres).
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.[9]
The Norfolk & Western Railroad in America came up with a novel way of not using track pans. They found certain grades were "momentum" grades. If the train stopped to take on water, the train had to have a helper engine to make it up the corresponding grade. If the train did not have to stop for water, then the "momentum" of the train would help it make the corresponding grade. Norfolk & Western added auxiliary water tenders, cars that only contained water. By carrying extra water the train did not have to stop for water, and did not need a helper. Norfolk & Western eliminated several helper districts as a result of using auxiliary water tenders.
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