If a person is On The Wagon they are no longer drinking alcohol. In the first part of the twentieth century, in the USA, water carts and wagons began carrying drinking water, or water for cleaning the streets. A person known to have given up alcohol could often be found waiting for the water wagon to arrive in order that they could quench their thirst. When the wagon would arrive, people would flock around it, and reformed heavy drinkers were said to actually travel on the wagon around town in order to take on as much water as possible and help suppress the craving for alcohol. Many people, including criminals who’d blamed alcohol for their crimes, were encouraged to sign a oath that they were “on the water wagon”. This meant they would rather drink water from the wagon than whisky. The expression soon spread into wider use.