A long trough filled with water, lying along a stretch of railroad track between the rails.
To explain the purpose of these, a little digression is in order. A steam locomotive not only requires a large supply of fuel (normally coal or oil), but also water. While it is possible to recycle the exhaust steam in a condenser to turn it back into water for reuse, the equipment to do so is heavy, bulky and adds complexity - all things the operator of steam locomotives tries to avoid. Condensing locomotives were in general only considered in desert regions, and even then the success was mixed. It proved easier and cheaper to just discard the exhaust steam, using its remaining energy to draught the fire.
Water is heavy, though, and a large, modern locomotive goes through a lot of it very quickly. Many large later locomotives weighed over a million pounds when fully fuelled and watered - half or more of that was the fuel and water in the tender. Even then, on a moderately long run - New York to Chicago, say - several stops would have to be made for water and fuel.
This obviously wastes time, and with competition intense among railroads, reducing the number of stops cuts your overall journey time without having to improve your track, locomotives etc. But carrying all the fuel and water needed for a long run all the way would cut heavily 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 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. Track pans were expensive to keep up, 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.