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Wednesday, August 24, 2016

The Secret History of the Atmospheric Railway and the Pneumatic Tube

Info pulled from the Usenet. Air (atmosphere) Railway Systems.

Today and Yesterday


The ultimate responsibility for this thread :-) belongs to George Medhurst (1759-1827), of England.

During a period of a few years about 1810, he invented three distinct forms of air-propelled transport. None of them was implemented during his lifetime; but all of them saw use eventually, reaching their greatest extent in the reverse order of their original invention. Medhurst's first method involved moving air through a tube a few inches in diameter, pushing a capsule along it; this simple idea was the pneumatic dispatch tube. Next he realized that if the same system was built much larger, it could carry passengers (or freight items larger than letters); it was natural to run the vehicle on tracks, and so this became known since the vehicle would be large enough to require tracks, this became known as a pneumatic railway.

But would anyone actually want to ride along mile after mile inside an opaque pipe?

Not likely. So he then thought of having only a piston moving within the pipe, somehow dragging along a vehicle outside it. He proposed several versions of this idea; in most of them the vehicle ran on rails, so this became known as an atmospheric railway (though a distinction between that term and the pneumatic railway was not always observed). The key feature of all versions of the system was a longitudinal valve: some sort of flexible flap running the length of the pipe, which would be held closed by air pressure except when the piston was actually passing. Medhurst did try to raise capital to implement this system, but failed. Now, while the first operable steam locomotive was built about 1804, steam-powered trains did not see regular use for passengers for some 25 years after that.

It was in the 1830's and 1840's that the steam railway was shown to be practical in both engineering and financial senses. But the same technical developments that made possible the practical steam railway also made the atmospheric railway, if not certainly practical, at least worth a try. And it offered the prospect of considerable advantages. Since the trains wouldn't have to carry their prime mover, they would be lighter; therefore the track could be built cheaper, and the trains' performance would be better. The trains wouldn't trail smoke wherever they went (and into the passenger cars in particular), and they would also be quiet. And if one section of the route was hilly and required more motive power, all that were needed would be more or larger pumping stations along that section; no need to add extra locomotives.

In short, very much the same advantages that electricity gave a few decades later. (Plus one more: a derailed train would tend to be kept near the track by the pipe and piston.) The success of the 1830's railways gave rise to the Railway Mania of the 1840's, when interest in railway shares reached absurd levels. In that climate the proposers of atmospheric lines could find the backing they needed, and four atmospheric lines opened in a period of about 3 years.

In order of opening, these were:

* The Dublin & Kingstown, from Kingstown to Dalkey in Ireland, 1.5 miles long; operated 1844-54.
* The London & Croydon, from Croydon to Forest Hill in London, England, 5 miles, then extended to New Cross for a total of 7.5 miles; operated 1846-47.
* The Paris a St-Germain, from Bois de Vezinet to St-Germain in Paris, France, 1.4 miles long; operated 1847-60.
* The South Devon, from Exeter to Teignmouth in Devonshire, England, 15 miles, then extended to Newton (now Newton Abbot), 20 miles altogether; operated 1847-48.

I note in passing that while I (as a fan of his) might like Isambard Kingdom Brunel to have invented the atmospheric system used on the South Devon, it is wrong to say that he did so. He did choose it and actively promoted it (well, "actively" is redundant with Brunel). It was actually developed by Samuel Clegg and Joseph and Jacob Samuda. Both of the longer, if shorter-lived, English lines used atmospheric propulsion in both directions of travel, whereas the French and Irish lines were built on hills and their trains simply returned downhill by gravity. Since all were single-track lines, the one-way system simplified the valves needed to let the pistons in and out of the pipes at their ends (possibly while traveling at speed).


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