The reservoir is communication with the boiler via the open tube d, which also serves as a steam dome as it allows the regulator g to be placed well above the boiler water level. When the valve f is opened water, which has been heated by the steam on top of it, passes from the reservoir to the boiler.
It is hard to see the point of this. Certainly the reservoir would store some extra heat, but the same result could be obtained with a slightly bigger boiler. Obviously increasing the boiler size of an existing locomotive requires a major rebuild, and if you have a locomotive not quite up to its work, putting a tank on top might have seemed an attractive alternative, but it didn't work out. Apart from the reported problems of mud and scale, there do however seem to be some definite snags:
1) The total steam-containing surface area is increased, requiring extra lagging.
2) A water-level gauge is required on the end of the reservoir, and has to be watched. An extra valve is required to control the flow of water from the reservoir to the boiler. Railwaymen rarely welcomed the prospect of dealing with more complexity for the same wages.
3) If the reservoir were to contain a lot of water, this would raise the locomotive's centre of gravity, with possibly bad effects on its stability.