Robert Stephenson's Planet class locomotive was the first true design of mainline express passenger locomotive. Delivered less than a year after Rocket it was one of the most successful early locomotive designs. Planet set the mold for British locomotive design for more than the next century featuring a multi tubular boiler; inside cylinders; crank axle; and the first use of proper frames.

The Planet class, and its 0-4-0 Samson derivative, found use across Britain with examples being supplied to railways in London and Glasgow. The Planet class proved popular in Europe too with examples being first exported and then built in France. Two were exported to Austria, and the first locomotive to steam in Russia was based on the design. Planet and Samson also crossed the Atlantic with more examples being built in the United States than in Europe.

A working replica of the revolutionary design was built in Manchester in 1992: the first mainline express passenger steam locomotive to be built in Britain since the 1960s.

This book outlines the technical design of the Planet and Samson locomotive, and charts the careers of the class members at home and abroad.