The 1884 machine was apparently born when a wealthy entrepreneur, the Comte de Dion, spied a model steam engine in a toy shop and tracked down its builders, Georges Bouton and Charles-Armand Trepardoux. Despite his family's concerns that he might be, um, mentally running on empty, the Comte persevered in bringing about a joint development project and the rest, as they say, is history.

According to RM Auctions, the Dos-A-Dos was christened "La Marquise" after the Comte's mother. It also participated in the first automobile race in 1887, and is capable of travelling about 30km on a tank of water and a combustible; it's capable of reaching a top speed of about 60km/h.

Credit: RM Auctions

Bouton and Trepardoux faced their share of challenges while engineering this sweet ride. According to the RM website:

The problem with steam-powered vehicles was that efficient boilers were huge, and powered locomotives and steamships. So how could one be miniaturised?

The two started off by adding a steam engine to a tricycle, and then built a Victoria quadricycle in 1883. This had belt drive and inconvenient rear-wheel steering, and its liquid fuel was prone to suddenly catching fire. With its large vertical boiler up front, it looked like a coffee pot on wheels, so back to the drawing board they went. A year later, they came up with a much more practical arrangement, which is the car offered today...

This quadricycle is much more compact, steering with its front wheels and driving the back wheels through connecting rods, rather like a locomotive. (The same principle was applied to the contemporary Hilderbrand & Wolfmuller motorcycle, though it proved difficult to ride, with so much unbalanced weight whizzing around.)

Credit: RM Auctions

At an RM Auctions event in Pennsylvania on Friday, bidding quickly flew away from the starting bid of US$500,000, jumping straight to US$1 million. From there, the bids rose, until finally settling on US$4.2 million — with a 10 per cent buyer's premium bringing it up to US$4.62 million. The auction house had estimated $2-2.5 million.

The winning bidder is only the fifth person to own the 127-year-old vehicle.

Below is a great little movie put together by RM Auctions that shows the car in action.