The iconic shape of George and Robert Stephenson's Rocket, as unveiled to the world in 1829, is arguably the most enduring silhouette in railway history. But why was Rocket that special, curious, shape? And why does the surviving locomotive, a star exhibit at London's Science Museum, look so unlike the striking yellow image associated with the Rocket today?

Rocket was built to take part in The Rainhill Trials, the competition to find a locomotive design to pull trains on the world's first passenger line, the Liverpool and Manchester. The trials caught the public's imagination and its...