Even in these days of electric trains, the popular idea of rail travel is dominated by a simple image - an engine pulling carriages along a track. And so fondly do we look back on the steam locomotive that it's hard to imagine a time when these fiery machines were regarded as dangerous monsters that spread fire and filled the air with filth. That's why early railway engineers were excited about any system that put the power source in an engine house at a safe distance from passengers.
Such a system was the atmospheric railway - so quaintly named because it relied on air pressure to move the carriages. By the 1840s, pneumatic tubes were being used to propel messages from one part of a building to another, and these worked on a simple principle. The message was placed in a container which fitted tightly into a pipe. When the air was sucked from the pipe in front of the container, atmospheric pressure behind it would force it through the pipe. To move a train using the same principle, it was necessary to run a vacuum pipe between the tracks. The message container was replaced with a metal piston, and this was connected by a rod to the carriages above.
Three such railways were built - from Dublin to Dalkey, from Exeter to Plymouth and from London to Croydon. Engine sheds and vacuum equipment cost half a million pounds for the Devon line alone. But the lines were a disaster, not because it was a bad idea, but because the materials available at that time were not up to the job.
The rod connecting the train to the piston had to pass through a slot in the top of the pipe, and this slot was sealed with a leather flap.
To keep it supple, the leather was coated in tallow. Unfortunately, the tallow attracted rats, which nibbled the leather and broke the seal. On more than one occasion, passengers were obliged to push their own trains. Not surprisingly, they preferred to take their chances with a steam engine.