Timothy Hackworth's entry in the Liverpool and Manchester Railway's famous Rainhill Trials of steam locomotives in 1829. The name means, roughly, 'Without equal' in French.
While a capable locomotive for the day, its technology was somewhat antiquated compared to George and Robert Stephenson's Rocket, the winner of the Rainhill Trials and the £500 prize money. Instead of the fire tube boiler of the Rocket, the Sans Pareil had a single flue tube. To increase the heating surface area, the flue passed in a U shape within the boiler; the firebox and chimney (stack in American parlance) were both positioned at the same end, one on either side.
The Sans Pareil had two cylinders, mounted vertically at the opposite end to the chimney, and driving one pair of driving wheels directly - the other pair were driven via connecting rods, in the typical steam locomotive fashion.
At the Rainhill Trials, Sans Pareil performed satisfactorily, though not exceptionally, until it was put out of contention because of a broken feed pump. After the trials, the Liverpool and Manchester Railway bought the locomotive as well as Rocket.