What does "on the wagon" mean?

Abstaining from drinking alcoholic beverages, as in Don't offer her wine; she's on the wagon. This expression is a shortening of on the water wagon, referring to the horse-drawn water car once used to spray dirt roads to keep down the dust. Its present meaning dates from about 1900. The antonym off the wagon, used for a resumption of drinking, dates from the same period. B.J. Taylor used it in Extra Dry (1906): “It is better to have been on and off the wagon than never to have been on at all.”