From the days in the UK when buses had a conductor as well as driver. The former sold tickets; monitored capacity. "Ding ding" was the signal from the conductor that it was OK to drive on. (Passenger door was at the rear.)
Too much information?
48 posted on 04/14/2022 5:04:10 PM PDT by Churchillspirit
(9/11/2001 and 9/11/2012: NEVER FORGET.)