A track pan is a long trough filled with water, lying along a stretch of railroad track between the rails.
Steam locomotives use large amounts of fuel but even greater amounts of water. While condensing equipment can be used in stationary applications, the bulk and complexity of these renders them impractical for locomotive use. Thus, there is no recycling of water in the average steam locomotive, and it must be constantly replenished from an onboard supply.
As trains and locomotives grew ever larger, the thirst of locomotives began to be a problem. Railroads began to desire the ability to run non-stop for very long distances, and locomotive tenderss grew enormous, up to a weight of half a million pounds fully laden. Even then, many stops would have to be made on a run such as New York to Chicago.
As competition increased, these delays became increasingly undesirable. Tenders could have grown still further, of course, but hauling a dead weight of water the entire length of the journey would hugely cut into revenue-earning capacity.
The obvious step is to take on supplies on the move. Fuelling on the move is obviously not a good idea, but water on the other hand is pretty harmless stuff, who cares if you spill a bunch of it? Thus the idea of the track pan and water scoop was born.
A scoop is fitted to the underside of the locomotive's tender in such a way as it can be raised or lowered, normally remotely (often by compressed air). The scoop feeds into a vertical pipe that has an outlet at the top of the tender's water tank. When the scoop is lowered at speed into a water-filled track pan, water is forced up the pipe and into the water tank.
While a train might have to slow down to pick up water in this fashion (too high a speed would result in water being forced too hard up into the tender, blowing the top or sides off), slowing down to 40mph or so is less painful than having to stop and fill up. The New York Central's famous Niagara locomotives had tenders fitted with special overflow pipes and vents to allow them to safely pick up water at 80mph.
Filling up from a track pan was certainly a messy affair, resulting in a huge spray of water in all directions. It would certainly be advised not to have a window open in the first few cars after the locomotive—fortunately in the US, those cars were normally for baggage or mail.
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 England, they were used by the LNER. No doubt other rail systems elsewhere used them too.
It was easy to distinguish a locomotive owned by a road that favored the track pan—the tender would have a huge coal capacity but a relatively tiny water tank. This was especially notable on the New York Central's later locomotives.
