The tank wagon was the main method of transporting liquid cargo in the early days of the railway. Instead of loading liquids into drums or barrels and then having to load these on and off closed or open vans at each side of a journey, the tank wagon could be filled with the fluid of choice of the operator and then emptied at the other end. This also increased the carrying capacity of each wagon, as a wagon with a 12-tonne weight limit could carry more fluid if it did not have to factor in the weight of many barrels on top of their contents. This type of tank wagon was more commonly seen hauling hazardous liquid chemicals, with different designs for milk and other such cargo.