Happiness is...a double bell (let's call it ding-ding)

The picture of a double-decker tram(Ding Ding); from a rainy afternoon in Hong Kong.

The tram system in Hong Kong has been around for more than a 100 years and is one of the three tramways in the world that use double deck cars. (And it is the only place where all the trams are double decked). 'Ding ding' started off  being the onomatopoeia of the iconic double bell ring that trams use, but now it is used to refer to the entire tramways. This is one of the earliest forms of public transport in Hong Kong (now it is also a tourist attraction, the most eco-friendly and economic way to get around town, possibly the only non-air-conditioned public transport in use here).