Alberta Railway Museum, this 1919 Gibbons water tank, designed to supply water to steam trains, can hold as much as ten home swimming pools. Though some railway water tanks held as little as 5000 gallons, the ones along the prairie lines usually had capacities between 40,000 and 60,000 gallons.
The water was held in the upper portion of the tank and a stove in the bottom had to be fired in winter to make sure it didn't freeze. The rod on top has a ball attached to a float. This showed the water level at a glance.
This basic style lasted through the forties, but each line used a slightly different building shape. This typical CN tank has sloped walls. CP tanks had vertical walls, often topped by a cupola.