Atmospheric Road sign, Dalkey
— Street sign, Dalkey, Ireland, 2018

In 2018 and 2019 I visited the sites of the four atmospheric railways built in the 1840s.

Most people have never heard of atmospheric railways. That's understandable. It's been more than a century and a half since the any of them existed. Before we go exploring let me give you a quick summary.

The very oldest railways were operated by horse or cable power, most often to carry stone from mines to water transportation. In 1829 the Liverpool and Manchester Railway, under construction, held the Rainhill Trials to decide whether to use the recently invented steam locomotive instead of cable. George Stephenson's Rocket was the only one to complete the trials successfully, pulling a train of 13 tons at 30 miles per hour on a mile of level track. In retrospect it was the start of a century of standard railway motive power.

But compared to earlier methods locomotives required a much more substantial roadbed and track to cope with their weight and the side to side "hunting" movement caused by the pistons. A locomotive used a good amount of its power just to move its own weight, and it slipped on grades as slight as one per cent. Some inventors kept thinking about the relative advantage of wayside engine houses transmitting power to a lightweight train. Today electricity is the answer, but its implementation was far in the future. Cable was known to work, but its maximum speed of not much over ten miles per hour had quickly been surpassed by locomotives.

The new idea was air pressure. Put a rail car in a tube, and use an engine to exhaust air in front of it, and normal atmospheric pressure would push it forward. A short demonstration was built at Brighton and it proved that the method worked. But would people want to ride inside a dark tube? Maybe not. Suppose instead you lay a pipe between the rails of a normal railway, and use air pressure to move a piston in the pipe, and connect the piston to an ordinary train.

The catch was how to connect the piston and the train. The pipe obviously would need a slot along its length with a linear valve which would open as the piston passed with its arm reaching up to the train but which would otherwise stay securely shut. Designing that was a problem. So was calculating the right diameter of pipe based on grades and distance, and so was calculating the engine power needed to evacuate air from a pipe that would inevitably have some amount of leakage.

Four atmospheric railways were built and put in regular passenger service. All four railways are still in use: three now use electric power and one (South Devon) uses diesel engines on board. You could go ride on them. I did.

Dublin and Kingstown Railway, Ireland: atmospheric 1844-1854,
from Dún Laoghaire to Dalkey.

London and Croydon Railway, England: atmospheric 1846-1847,
from New Cross Gate to West Croydon.

Chemin de fer de Paris à Saint-Germain, France: atmospheric 1847-1860,
from Le Vésinet Le Pecq to Saint-Germain-en-Laye.

South Devon Railway, England: atmospheric 1847-1848,
from Exeter to Newton Abbot.