South Devon Railway Comet Class

The South Devon Railway Comet Class refers to the locomotives, both atmospheric and steam, that operated on Isambard Kingdom Brunel's South Devon Railway in the late 1840s — a railway that occupies a unique and slightly melancholy place in British transport history as the last and most ambitious attempt to operate a main-line railway using the atmospheric traction system before steam locomotive working was conclusively proved superior. The atmospheric railway — in which trains were propelled by atmospheric pressure acting on a piston in a continuous pipe laid between the rails, with the air exhausted ahead of the piston by stationary pumping engines — was an intriguing concept that Brunel adopted with characteristic boldness for the South Devon Railway's main line from Exeter to Plymouth in 1844–48.

The atmospheric system worked on the South Devon Railway for a period in 1847–48, with trains achieving impressive speeds, but the fundamental problem was the leather valve that sealed the slot through which the piston rod connected the pipe to the train: the leather deteriorated rapidly in the South Devon's coastal climate, rats gnawed it, and maintaining the airtight seal across many miles of pipeline proved impossible in practice. By 1848 Brunel was forced to acknowledge failure and convert the line to conventional steam locomotive working. The Comet Class locomotives — broad-gauge 4-4-0 saddle-tank engines built by various manufacturers to work the SDR — then took over the working of what became the GWR's main line to Cornwall, operating on Brunel's 7 ft broad gauge until the final broad-gauge conversion of 1892.

The South Devon Railway's atmospheric episode is one of the great Victorian engineering stories — a genuinely brilliant concept that failed in practice, costing the railway enormous sums. None of the Comet Class locomotives was preserved, though the atmospheric railway's history is well documented at the Didcot Railway Centre and through the National Railway Museum's collections.