Wagon pooling came about because the pre-war system had failed during the first years of the war. Not only was there an increase in traffic due to the rates for sea-bourn carriage increasing, but there was massive misuse of wagons by the military. For instance, RCH returns for January 1916 showed that Government departments held 5000 wagons for an aggregate of 50000 days. It was also common for wagons to be sent on unnecessary journeys, sometimes of only a few miles, and using a whole wagon for minimal loads.
Much of this information came from "Britains Railways in World War 1" by J.A.B. Hamilton, published 1967.