Atmospheric railway

An atmospheric railway uses differential air pressure to provide power for propulsion of a railway vehicle. A static power source can transmit motive power to the vehicle in this way, avoiding the necessity of carrying mobile power generating equipment. The air pressure, or partial vacuum can be conveyed to the vehicle in a continuous pipe, where the vehicle carries a piston running in the tube. Some form of re-sealable slot is required to enable the piston to be attached to the vehicle. Alternatively the entire vehicle may act as the piston in a large tube or be coupled electromagnetically to the piston.
Several variants of the principle were proposed in the early 19th century, and a number of practical forms were implemented, but all were overcome by unforeseen disadvantages and discontinued within a few years.
A modern proprietary system has been developed and is in use for short-distance applications. Porto Alegre Metro airport connection in Porto Alegre, Brazil, is one of them.

History

In the early days of railways, single vehicles or groups were propelled by men or horses. As mechanical power came to be understood, locomotive engines were developed; the iron horse. These had serious limitations; in particular, being much heavier than the wagons in use, they frequently broke the rails. Also, lack of adhesion at the iron-to-iron wheel-rail interface was a limitation, for example in trials on the Kilmarnock and Troon Railway.
Many engineers turned their attention to transmitting power from a static power source, a stationary engine, to a moving train. Such an engine could be more robust and with more available space, potentially more powerful. The solution to transmitting the power, before the days of practical electricity, was the use of either a cable system or air pressure.

Medhurst

In 1799, George Medhurst of London discussed the idea of moving goods pneumatically through cast iron pipes, and in 1812, he proposed blowing passenger carriages through a tunnel.
Medhurst proposed two alternative systems: either the vehicle itself was the piston, or the tube was relatively small with a separate piston. He never patented his ideas and they were not taken further by him.

19th century

Vallance

In 1824, John Vallance patented an atmospheric railway which consisted of a tube sufficiently large to allow carriages with passengers and goods to pass through them. The tube was to be formed from cast-iron cylinders and extend from town to town, with powerful exhausting machinery at one end. The cylindrical carriages would be just less than the inside diameter of the tube and act as pistons, moving under the force of air entering the tube at the open end as air at the other end was pumped out. In 1826 Vallance built a model of the system at his premises in Brighton with a diameter tube. Rails were cast in to the lower part of the tube and bear skin was used to seal the annular space between the tube and the carriage. Braking was accomplished by opening doors at each end of the vehicle. Vallance's system worked, but the scheme was considered uneconomic both because of its costs and because prospective passengers would be repelled by being enclosed in a tube.

Pinkus

In 1835, Henry Pinkus patented a system with a square section tube with a low degree of vacuum, limiting leakage loss. He later changed to a small-bore vacuum tube. He proposed to seal the slot that enabled the piston to connect with the vehicle with a continuous rope; rollers on the vehicle lifted the rope in front of the piston connection and returned it afterwards.
He built a demonstration line alongside the Kensington Canal, and issued a prospectus for his National Pneumatic Railway Association. He was unable to interest investors, and his system failed when the rope stretched. However, his concept, a small bore pipe with a resealable slot was the prototype for many successor systems.

Samuda and Clegg

Wormwood Scrubs scheme

were shipbuilders and engineers, and owned the Southwark Ironworks; they were both members of the Institution of Civil Engineers. Samuel Clegg was a gas engineer and they worked in collaboration on their atmospheric system. About 1835, they read Medhurst's writings, and developed a small bore vacuum pipe system. Clegg worked on a longitudinal flap valve, for sealing the slot in the pipe.
In 1838, they took out a patent "for a new improvement in valves" and built a full-scale model at Southwark. In 1840, Jacob Samuda and Clegg leased half a mile of railway line on the West London Railway at Wormholt Scrubs, where the railway had not yet been opened to the public. In that year Clegg left for Portugal, where he was pursuing his career in the gas industry.
Samuda's system involved a continuous cast iron pipe laid between the rails of a railway track; the pipe had a slot in the top. The leading vehicle in a train was a piston carriage, which carried a piston inserted in the tube. It was held by a bracket system that passed through the slot, and the actual piston was on a pole ahead of the point at which the bracket left the slot. The slot was sealed from the atmosphere by a continuous leather flap that was opened immediately in advance of the piston bracket and closed again immediately behind it. A pumping station ahead of the train would pump air from the tube, and atmospheric pressure behind the piston would push it forward.
The Wormwood Scrubs demonstration ran for two years. The traction pipe was of 9 inches diameter, and a 16 hp stationary engine was used for power. The gradient on the line was a steady 1 in 115. In his treatise, described below, Samuda implies that the pipe would be used in one direction only, and the fact that only one pumping station was erected suggests that trains were gravitated back to the lower end of the run after the atmospheric ascent, as was later done on the Dalkey line. Many of the runs were public. Samuda quotes the loads and degree of vacuum and speed of some of the runs; for example:

11 June 1840: 11 tons 10 cwt; maximum speed 22.5 mph; 15 inches of vacuum
10 August 1840: 5 tons 0 cwt; maximum speed 30 mph; 20 inches of vacuum.
Competing solutions

There was enormous public interest in the ideas surrounding atmospheric railways, and at the same time as Samuda was developing his scheme, others put other ideas forward.

Nickels and Keane propelled trains by pumping air into a continuous canvas tube. The trains had a pair of pinch rollers squeezing the outside of the tube, and the air pressure forced the vehicles forward. The effect was the converse of squeezing a toothpaste tube. They claimed a successful demonstration in a timber yard in Waterloo Road.
James Pilbrow proposed a loose piston fitted with a toothed rack. Cog wheels would be turned by it, and they were on spindle passing through glands to the outside of the tube. The leading carriage of the train would have a corresponding rack and be impelled forward by the rotation of the cog wheels. Thus the vehicle would keep pace with the piston exactly, without any direct connection to it.
Henry Lacey conceived a wooden tube, made by coopers as a long, continuous barrel with the opening slot and a timber flap retained by an india-rubber hinge.
Clarke and Varley proposed sheet iron tubes with a continuous longitudinal slit. If the tubes were made to precision standards, the vacuum would keep the slit closed, but the piston bracket on the train would spring the slit open enough to pass The elasticity of the tube would close it again behind the piston carriage.
Joseph Shuttleworth suggested a hydraulic tube where water pressure rather than a partial atmospheric vacuum would propel the train. In mountainous areas where plentiful water was available, a pumping station would be unnecessary: the water would be used directly. Instead of the flap to seal the slot in the tube, a continuous shaped sealing rope, made of cloth impregnated with india-rubber would be within the pipe. Guides on the piston would lift it into position and the water pressure would hold it in place behind the train. Use of a positive pressure enabled a greater pressure differential than a vacuum system. However, the water in the pipe would have to be drained manually by staff along the pipe after every train.
Samuda's treatise

In 1841, Joseph Samuda published A Treatise on the Adaptation of Atmospheric Pressure to the Purposes of Locomotion on Railways.
It ran to 50 pages, and Samuda described his system; first the traction pipe:

The moving power is communicated to the train through a continuous pipe or main, laid between the rails, which is exhausted by air pumps worked by stationary steam engines, fixed on the road side, the distance between them varying from one to three miles, according to the nature and traffic of the road. A piston, which is introduced into this pipe, is attached to the leading carriage in each train, through a lateral opening, and is made to travel forward by means of the exhaustion created in front of it. The continuous pipe is fixed between the rails and bolted to the sleepers which carry them; the inside of the tube is unbored, but lined or coated with tallow 1/10th of an inch thick, to equalize the surface and prevent any unnecessary friction from the passage of the travelling piston through it.

The operation of the closure valve was to be critical:

Along the upper surface of the pipe is a continuous slit or groove about two inches wide. This groove is covered by a valve, extending the whole length of the railway, formed of a strip of leather riveted between iron plates, the top plates being wider than the groove and serving to prevent the external air forcing the leather into the pipe when the vacuum is formed within it; and the lower plates fitting into the groove when the valve is shut, makes up the circle of the pipe, and prevents the air from passing the piston; one edge of this valve is securely held down by iron bars, fastened by screw bolts to a longitudinal rib cast on the pipe, and allows the leather between the plates and the bar to act as a hinge, similar to a common pump valve; the other edge of the valve falls into a groove which contains a composition of beeswax and tallow: this composition is solid at the temperature of the atmosphere, and becomes fluid when heated a few degrees above it. Over this valve is a protecting cover, which serves to preserve it from snow or rain, formed of thin plates of iron about five feet long hinged with leather, and the end of each plate underlaps the next in the direction of the piston's motion, thus ensuring the lifting of each in succession.

The piston carriage would open and then close the valve:

To the underside of the first carriage in each train is attached the piston and its appurtenances; a rod passing horizontally from the piston is attached to a connecting arm, about six feet behind the piston. This connecting arm passes through the continuous groove in the pipe, and being fixed to the carriage, imparts motion to the train as the tube becomes exhausted; to the piston rod are also attached four steel wheels, which serve to lift the valve, and form a space for the passage of the connecting arm, and also for the admission of air to the back of the piston; another steel wheel is attached to the carriage, regulated by a spring, which serves to ensure the perfect closing of the valve, by running over the top plates immediately after the arm has passed. A copper tube or heater, about ten feet long, constantly kept hot by a small stove, also fixed to the underside of the carriage, passes over and melts the surface of the composition which upon cooling becomes solid, and hermetically seals the valve. Thus each train in passing leaves the pipe in a fit state to receive the next train.

Entering and leaving the pipe was described:

The continuous pipe is divided into suitable sections by separating valves, which are opened by the train as it goes along: these valves are so constructed that no stoppage or diminution of speed is necessary in passing from one section to another. The exit separating valve, or that at the end of the section nearest to its steam engine, is opened by the compression of air in front of the piston, which necessarily takes place after it has passed the branch which communicates with the air-pump; the entrance separating valve, is an equilibrium or balance valve, and opens immediately the piston has entered the pipe. The main pipe is put together with deep socket joints, in each of which an annular space is left about the middle of the packing, and filled with a semi-fluid: thus any possible leakage of air into the pipe is prevented.

At that time railways were developing rapidly, and solutions to the technical limitations of the day were eagerly sought, and not always rationally evaluated. Samuda's treatise put forward the advantages of his system:

transmission of power to trains from static power stations; the static machinery could be more fuel efficient;
the train would be relieved of the necessity of carrying the power source, and fuel, with it;
power available to the train would be greater so that steeper gradients could be negotiated; in building new lines this would hugely reduce construction costs by enabling reducing earthworks and tunnels;
elimination of a heavy locomotive from the train would enable lighter and cheaper track materials to be used;
passengers, and lineside residents, would be spared the nuisance of smoke emission from passing trains; this would be especially useful in tunnels;
collisions between trains would be impossible, because only one train at a time could be handled on any section between two pumping stations; collisions were at the forefront of the mind of the general public in those days before modern signalling systems, when a train was permitted to follow a preceding train after a defined time interval, with no means of detecting whether that train had stalled somewhere ahead on the line;
the piston travelling in the tube would hold the piston carriage down and, Samuda claimed, prevent derailments, enabling curves to be negotiated safely at high speed;
persons on the railway would not be subjected to the risk of steam engine boiler explosions.

Samuda also rebutted criticisms of his system that had become widespread:

that if a pumping station failed the whole line would be closed because no train could pass that point; Samuda explained that a pipe arrangement would enable the next pumping station ahead to supply that section; if this was at reduced pressure, the train would nonetheless be able to pass, albeit with a small loss of time;
that leakage of air at the flap or the pipe joints would critically weaken the vacuum effect; Samuda pointed to experience and test results on his demonstration line, where this was evidently not a problem;
the capital cost of the engine houses was a huge burden; Samuda observed that the capital cost of steam locomotives was eliminated, and running costs for fuel and maintenance could be expected to be lower.