On the wagon
This expression means abstaining from alcohol and is American dating from the turn of the 19th/20th century. The expression began life as ‘on the water cart’ and then ‘on the water wagon’. Water carts and wagons were familiar sight in parts of America where running tap water was not readily available. Prohibition was a big issue in America at that time and being on the water cart or wagon was widely encouraged. Eventually, the ‘water’ bit was dropped and it became simply ‘on the wagon’. Its antonym, ‘off the wagon’ or ‘to fall off the wagon” signified a return to alcohol and dates from the same era.