Developing a practical scheme

Jacob and Joseph Samuda 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 (later renamed Wormwood 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 (jointed) 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 (below). Many of the runs were public. Samuda quotes the loads and degree of vacuum and speed of some of the runs; there seems to be little correlation; 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.^[4]

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.

A patent

In April 1844, Jacob and Joseph Samuda took out a patent for their system. Soon after this, Joseph Samuda died and it was left to his brother Jacob to continue the work. The patent was in three parts: the first describing the atmospheric pipe and piston system, the second describing how in areas of plentiful water supply, the vacuum might be created by using tanks of water at differing levels; and the third section dealt with level crossings of an atmospheric railway.^[2]

A steam railway at first

The London and Croydon Railway (L&CR) obtained its authorising Act of Parliament in 1835, to build its line from a junction with the London and Greenwich Railway (L&GR) to Croydon. At that time the L&GR line was under construction, and Parliament resisted the building of two railway termini in the same quarter of London, so that the L&CR would have to share the L&GR's London Bridge station. The line was built for ordinary locomotive operation. A third company, the London and Brighton Railway (L&BR), was promoted, and it too had to share the route into London by running over the L&CR.

When the lines opened in 1839, it was found that congestion arose due to the frequent stopping services on the local Croydon line; this was particularly a problem on the 1 in 100 ascent from New Cross to Dartmouth Arms.^[3] The L&CR engineer, William Cubitt, proposed a solution to the problem: a third track would be laid on the east side of the existing double track main line, and all the local trains in both directions would use it. The faster Brighton trains would be freed of the delay following a stopping train. Cubitt had been impressed during his visit to the Dalkey line, and the new L&CR third track would use atmospheric power. The local line would also be extended to Epsom, also as a single track atmospheric line. These arrangements were adopted and Parliamentary powers obtained on 4 July 1843, also authorising a line to a terminal at Bricklayers Arms. Arrangements were also made with the L&GR for them to add an extra track on the common section of their route. On 1 May 1844, the Bricklayers Arms terminus opened, and a frequent service was run from it, additional to the London Bridge trains.^[2][3][12]

Now atmospheric as well

The L&CR line diverged to the south-west at Norwood Junction (then called Jolly Sailor, after an inn), and needed to cross the L&BR line. The atmospheric pipe made this impossible on the flat, and a flyover was constructed to enable the crossing: this was the first example in the railway world.^[13] This was in the form of a wooden viaduct with approach gradients of 1 in 50. A similar flyover was to be built at Corbetts Lane Junction, where the L&CR additional line was to be on the north-east side of the existing line, but this was never made.

A 15-inch diameter traction pipe was installed between Forest Hill (then called Dartmouth Arms, also after a local inn) and Croydon (later renamed West Croydon. Although Samuda supervised the installation of the atmospheric apparatus, a weather flap, a hinged iron plate that covered the leather slot valve in the Dalkey installation, was omitted. The L&CR had an Atmospheric Engineer, James Pearson. Maudslay, Son and Field supplied the three 100 hp steam engines and pumps at Dartmouth Arms, Jolly Sailor and Croydon, and elaborate engine houses had been erected for them. They were designed in a gothic style by W H Brakespear, and had tall chimneys which also exhausted the evacuated air at high level.^{[note 4]}

A two-needle electric telegraph system was installed on the line, enabling station staff to indicate to the remote engine house that a train was ready to start.

This section, from Dartmouth Arms to Croydon, began operation on the atmospheric system in January 1846.

The traction pipe slot and the piston bracket were handed; that is the slot closure flap was continuously hinged on one side, and the piston support bracket was cranked to minimise the necessary opening of the flap. This meant that the piston carriage could not simply be turned on a turntable at the end of a trip. Instead it was double ended, but the piston was manually transferred to the new leading end. The piston carriage itself had to be moved manually (or by horse power) to the leading end of the train. At Dartmouth Arms the station platform was an island between the two steam operated lines. Cubitt designed a special system of pointwork that enabled the atmospheric piston carriage to enter the ordinary track.^{[note 5]}

The Board of Trade inspector, General Pasley, visited the line on 1 November 1845 to approve it for opening of the whole line. The Times newspaper reported the event. A special train left London Bridge hauled by a steam locomotive; at Forest Hill the locomotive was detached and:

the piston carriage substituted and the train thence became actuated by atmospheric pressure. The train consisted of ten carriages (including that to which the piston is attached) and its weight was upward of fifty tons. At seven and a half minutes past two the train left the point of rest at the Dartmouth Arms, and at eight and three-quarter minutes past, the piston entered the valve,^{[note 6]} when it immediately occurred to us that one striking advantage of the system was the gentle, the almost imperceptible, motion on starting. On quitting the station on locomotive lines we have frequently experienced a "jerk" amounting at times to an absolute "shock" and sufficient to alarm the nervous and timid passenger. Nothing of the sort, however, was experienced here. Within a minute and a quarter of the piston entering the pipe, the speed attained against a strong headwind was at the rate of twelve miles an hour; in the next minute, viz. at eleven minutes past two, twenty-five miles an hour; at thirteen minutes past two, thirty-four miles an hour; fourteen minutes past two, forty miles an hour; and fifteen minutes past two, fifty-two miles an hour, which was maintained until sixteen minutes past two, when the speed began to diminish, and at seventeen and a half minutes past two, the train reached the Croydon terminus, thus performing the journey from Dartmouth Arms, five miles, in eight minutes and three-quarters. The barometer in the piston carriage indicated a vacuum of 25 inches and that in the engine house a vacuum of 28 inches.^{[note 7][14]}

The successful official public run was widely reported, and immediately new schemes for long-distance railways on the atmospheric system were being promoted; the South Devon Railway's shares appreciated overnight.

Opening

Pasley's report of 8 November was favourable, and the line was clear to open. The directors hesitated, desiring to gain a little more experience beforehand. On 19 December 1845 the crankshaft of the Forest Hill stationary engine fractured, and the engine was unusable. However, the part was quickly replaced and on 16 January 1846, the line opened.

At 11:00 that morning, the crankshaft of one of the Croydon engines broke. Two engines had been provided, so traffic was able to continue using the other,^{[note 8]} until at 7:20 p.m. that engine suffered the same fate. Again repairs were made until 10 February 1846, when both of the Croydon engines failed.

This was a bitter blow for the adherents of the atmospheric system; shortcomings in the manufacture of the stationary engines procured from a reputable engine-maker said nothing about the practicality of the atmospheric system itself, but as Samuda said to the Board:

"The public cannot discriminate (because it cannot know) the cause of the interruptions, and every irregularity is attributed to the atmospheric system."^[15]

Two months later, the beam of one of the Forest Hill engines fractured. At this time the directors were making plans for the Epsom extension; they quickly revised their intended purchase of engines from Maudslay, and invited tenders; Boulton and Watt of Birmingham were awarded the contract, their price having been considerably less than their competitors'.

Amalgamation

The London and Brighton Railway amalgamated with the L&CR on 6 July 1846, forming the London, Brighton and South Coast Railway (LB&SCR). For the time being the directors of the larger company continued with the L&CR's intentions to use the atmospheric system.

Technical difficulties

The summer of 1846 was exceptionally hot and dry, and serious difficulties with the traction pipe flap valve started to show themselves. It was essential to make a good seal when the leather flap was closed, and the weather conditions made the leather stiff. As for the tallow-and-beeswax compound that was supposed to seal the joint after every train, Samuda had originally said "this composition is solid at the temperature of the atmosphere, and becomes fluid when heated a few degrees above it"^[4] and the hot weather had that effect. Samuda's original description of his system had included a metal weather valve that closed over the flap, but this had been omitted on the L&CR, exposing the valve to the weather, and also encouraging the ingestion of debris, including, an observer reported, a handkerchief dropped by a lady on to the track. Any debris lodging in the seating of the flap could only have reduced its effectiveness.

Moreover, the tallow – that is, rendered animal fat – was attractive to the rat population. An 1859 source reports rats entering the iron tube overnight to eat the tallow, and "hundreds" being killed each morning when the pump was activated for the first train.^[16] Delays became frequent, due to inability to create enough vacuum to move the trains, and stoppages on the steep approach inclines at the flyover were commonplace, and widely reported in the press.

The Directors now began to feel uneasy about the atmospheric system, and in particular the Epsom extension, which was to have three engines. In December 1846, they asked Boulton and Watt about cancelling the project, and were told that suspending the supply contract for a year would cost £2,300. The Directors agreed to this.

The winter of 1846/7 brought new meteorological difficulties: unusually cold weather made the leather flap stiff, and snow got into the tube^{[note 9]} resulting in more cancellations of the atmospheric service. A track worker was killed in February 1847 while steam substitution was in operation. This was tragically unfortunate, but it had the effect of widespread reporting that the atmospheric was, yet again, non-operational.^[17]

Sudden end

Through this long period, the Directors must have become less and less committed to pressing on with the atmospheric system, even as money was being spent on extending it towards London Bridge. (It opened from Dartmouth Arms to New Cross in January 1847, using gravitation northbound and the Dartmouth Arms pumping station southbound.) In a situation in which public confidence was important, the Directors could not express their doubts publicly, at least until a final decision had been taken. On 4 May 1847,^[18] the directors announced "that the Croydon Atmospheric pipes were pulled up and the plan abandoned".

The reason seems not to have been made public at once, but the trigger seems to have been the insistence of the Board of Trade inspector on a second junction at the divergence of the Brighton and Epsom lines. It is not clear what this refers to, and may simply have been a rationalisation of the timing of a painful decision. Whatever the reason, there was to be no more atmospheric work on the LB&SCR.^[2]

Getting authorisation

The Great Western Railway (GWR) and the Bristol and Exeter Railway working collaboratively had reached Exeter on 1 May 1844, with a broad gauge railway connecting the city to London. Interested parties in Devonshire considered it important to extend the connection to Plymouth, but the terrain posed considerable difficulties: there was high ground with no easy route through.

After considerable controversy, the South Devon Railway Company (SDR) obtained its Act of Parliament authorising a line, on 4 July 1844.

Determining the route

The Company's engineer was the innovative engineer Isambard Kingdom Brunel. He had visited the Dalkey line and he had been impressed with the capabilities of the atmospheric system on that line. Samuda had always put forward the advantages of his system, which (he claimed) included much better hill climbing abilities and lighter weight on the track. This would enable a line in hilly terrain to be planned with steeper than usual gradients, saving substantial cost of construction.

If Brunel had decided definitely to use the atmospheric system at the planning stage, it would have allowed him to strike a route that would have been impossible with the locomotive technology of the day. The route of the South Devon Railway, still in use today, has steep gradients and is generally considered "difficult". Commentators often blame this on it being designed for atmospheric traction; for example:

Sekon, describing the topography of the line, says that beyond Newton Abbot,

the conformation of the country is very unsuitable for the purpose of constructing a railway with good gradients. This drawback did not at the time trouble Mr. Brunel, the engineer to the South Devon Railway Company, since he proposed to work the line on the atmospheric principle, and one of the advantages claimed for the system being that steep banks were as easy to work as a level.^[19]

• The line "was left with a legacy of a line built for atmospheric working with the consequent heavy gradients and sharp curves".^[20]
• Brunel "seriously doubted the ability of any engine to tackle the kind of gradients which would be necessary on the South Devon".^[21]

In fact the decision to consider the adoption of the atmospheric system came after Parliamentary authorisation, and the route must have been finalised before submission to Parliament.

Eight weeks after passage of the Act, the shareholders heard that "Since the passing of the Act, a proposal has been received ... from Messrs. Samuda Brothers ... to apply their system of traction to the South Devon Line." Brunel and a deputation of the directors had been asked to visit the Dalkey line. The report went on that as a result,

In view of the fact that at many points of the line both the gradients and curves will render the application of this principle particularly advantageous, your directors have resolved that the atmospheric system, including an electric telegraph, should be adopted on the whole line of the South Devon Railway.^[22]

Construction and opening

Construction started at once on the section from Exeter to Newton Abbot (at first called Newton); this first part is broadly level: it was the section onwards from Newton that was hilly. Contracts for the supply of the 45 horsepower (34 kW) pumping engines and machinery were concluded on 18 January 1845, to be delivered by 1 July in the same year. Manufacture of the traction pipes ran into difficulties: they were to be cast with the slot formed,^{[note 10]} and distortion was a serious problem at first.

Delivery of the machinery and laying of the pipes was much delayed, but on 11 August 1846, with that work still in progress, a contract was let for the engines required over the hilly section beyond Newton. These were to be more powerful, at 64 horsepower (48 kW), and 82 horsepower (61 kW) in one case, and the traction pipe was to be of a larger diameter.

The train service started between Exeter and Teignmouth on 30 May 1846, but this was operated by steam engines, hired from the GWR. At length, on 13 September 1847^{[note 11]} the first passenger trains started operating on the atmospheric system.^[23][24] Atmospheric goods trains may have operated a few days previously.

Four atmospheric trains ran daily in addition to the advertised steam service, but after a time they replaced the steam trains. At first the atmospheric system was used as far as Teignmouth only, from where a steam engine hauled the train including the piston carriage to Newton, where the piston carriage was removed, and the train continued on its journey. From 9 November some atmospheric working to Newton took place, and from 2 March 1848, all trains on the section were atmospheric.

Through that winter of 1847-8 a regular service was maintained to Teignmouth. The highest speed recorded was an average of 64 mph (103 km/h) over 4 miles (6.4 km) hauling 28 long tons (28 t), and 35 mph (56 km/h) when hauling 100 long tons (100 t).^{[citation needed]}

Two significant limitations of the atmospheric system were overcome at this period. The first was an auxiliary traction pipe was provided at stations; it was laid outside the track, therefore not obstructing pointwork. The piston carriage connected to it by a rope—the pipe must have had its own piston—and the train could be hauled into a station and on to the start of the onward main pipe. The second development was a level crossing arrangement for the pipe: a hinged cover plate lay across the pipe for road usage, but when the traction pipe was exhausted, a branch pipe actuated a small piston which raised the cover, enabling the piston carriage to pass safely, and acting as a warning to road users. Contemporary technical drawings show the traction pipe considerably lower than normal, with its top about level with the rail heads, and with its centre at the level of the centre of the transoms. No indication is shown as to how track gauge was maintained.

Underpowered traction system

Although the trains were running ostensibly satisfactorily, there had been technical miscalculations. It seems^[25] that Brunel originally specified 12-inch (300 mm) for the level section to Newton and 15-inch (380 mm) pipes for the hilly part of the route, and in specifying the stationary engine power and vacuum pumps, he considerably underpowered them. The 12-inch (300 mm) pipes seem to have been scrapped, and 15-inch (380 mm) pipes installed in their place, and 22-inch (560 mm) pipes started to be installed on the hilly sections. Changes to the engine control governors were made to uprate them to run 50% faster than designed. It was reported that coal consumption was much heavier than forecast, at 3s 1½d per train mile instead of 1s 0d (and instead of 2s 6d which was the hire charge for the leased GWR steam locomotives). This may have been partly due to the electric telegraph not yet having been installed, necessitating pumping according to the timetable, even though a train might be running late. When the telegraph was ready, on 2 August, coal consumption in the following weeks fell by 25%.^[26]

Problems with the slot closure

During the winter of 1847–1848, the leather flap valve that sealed the traction pipe slot began to give trouble. During the cold days of winter, the leather froze hard in frost after saturation in rain. This resulted in its failing to seat properly after the passage of a train, allowing air into the pipe and reducing the effectiveness of pumping. In the following spring and summer there was hot and dry weather, and the leather valve dried out, with a similar outcome. Brunel had the leather treated with whale oil in an attempt to maintain flexibility. There was said to be a chemical reaction between iron oxide on the pipe and the tannin in the leather. There were also difficulties with the leather cup seal on the pistons.

Commentators observe that the South Devon system omitted the iron weather flap that was used on the Dalkey line to cover the flap valve. On that line, iron plates were turned away immediately ahead of the piston bracket. It is not recorded why this was omitted in South Devon, but at speed that arrangement must have involved considerable mechanical force, and generated environmental noise.

In May and June, even more serious trouble was experienced when sections of the flap tore away from its fixing, and sections had to be quickly replaced. Samuda had a contract with the company to maintain the system, and he advised installation of a weather cover, but this was not adopted. This would not have rectified the immediate problem, and complete replacement of the leather flap was required; this was estimated to cost £32,000—a very large sum of money then—and Samuda declined to act.

Abandonment

With a contractual impasse during struggles to keep a flawed system in operation, it was inevitable that the end was near. At a shareholders' meeting on 29 August 1848, the directors were obliged to report all the difficulties, and that Brunel had advised abandonment of the atmospheric system; arrangements were being made with the Great Western Railway to provide steam locomotives, and the atmospheric system would be abandoned from 9 September 1848.

Brunel's report to the Directors, now shown the meeting, was comprehensive, and he was also mindful of his own delicate position, and of the contractual obligations of Samuda. He described the stationary engines, obtained from three suppliers: "These engines have not, on the whole, proved successful; none of them have as yet worked very economically, and some are very extravagant in the use of fuel." As to the difficulties with the leather valve in extremes of weather, heat, frost and heavy rain,

The same remedies apply to all three, keeping the leather of the valve oiled and varnished, and rendering it impervious to the water, which otherwise soaks through it in wet weather, or which freezes it in cold, rendering it too stiff to shut down; and the same precaution prevents the leather being dried up and shrivelled by the heat; for this, and not the melting of the composition, is the principal inconvenience resulting from heat. A little water spread on the valve from a tank in the piston carriage has also been found to be useful in very dry weather, showing that the dryness, and not the heat, was the cause of the leakage.

But there was a much more serious problem: "A considerable extent of longitudinal valve failed by the tearing of the leather at the joints between the plates. The leather first partially cracked at these points, which caused a considerable leakage, particularly in dry weather. After a time it tears completely through."

Maintenance of the traction pipe and the valve was Samuda's contractual responsibility, but Brunel indicated that he was blaming the company for careless storage, and for the fact that the valve had been installed for some time before being used by trains; Brunel declined to go into the liability question, alluding to possible palliative measures, but concluded:

The cost of construction has far exceeded our expectations, and the difficulty of working a system so totally different from that to which everybody—traveller as well as workmen—is accustomed, have (sic) proved too great; and therefore, although, no doubt, after some further trial, great reductions may be made in the cost of working the portion now laid, I cannot anticipate the possibility of any inducement to continue the system beyond Newton.^[27]

Huge hostility was generated among some shareholders, and Samuda, and Brunel in particular were heavily criticised, but the atmospheric system on the line was finished.

Retention recommended

Thomas Gill had been Chairman of the South Devon board and wished to continue with the atmospheric system. In order to press for this he resigned his position, and in November 1848, he published a pamphlet urging retention of the system. He created enough support for this that an Extraordinary General Meeting of the Company was held on 6 January 1849. Lengthy technical discussion took place, in which Gill stated that Clark and Varley were prepared to contract to complete the atmospheric system and maintain it over a section of the line. There were, Gill said, twenty-five other inventors anxious to have their creations tried out on the line. The meeting lasted for eight hours, but finally a vote was taken: a majority of shareholders present were in favour of continuing with the system, 645 to 567 shares. However, a large block of proxies were held by shareholders who did not wish to attend the meeting, and with their votes abandonment was confirmed by 5,324 to 1,230.

That was the end of the atmospheric system on the South Devon Railway.

Rats

It is often asserted among enthusiasts' groups that one factor in the failure of the leather flap was rats, attracted to the tallow, gnawing at it. Although rats are said to have been drawn into the traction pipe in the early days, there was no reference to this at the crisis meeting described above. Historian Colin Divall believes there to be "no documentary evidence whatsoever" for rats causing such problems on the railway.^[28]

Wormwood Scrubs demonstration line

The piston carriage on the demonstration line was an open four-wheeled track. No controls of any kind are shown on a drawing. The beam that carried the piston was called the "perch", and it was attached directly to the axles, and pivoted at its centre point; it had a counterweight to the rear of the attachment bracket (called a "coulter").

Dalkey line

The customary train consist was two coaches, the piston carriage, which included a guard's compartment and third class accommodation, and a second class carriage, with end observation windows at the rear. There was no first class carriage. The guard had a screw brake, but no other control. Returning (descending) was done under gravity, and the guard had a lever which enabled him to swing the piston assembly to one side, so that the descent was made with the piston outside the tube.

Saint Germain line

The section put into service, Le Pecq to Saint Germain, was almost exactly the same length as the Dalkey line, and was operated in a similar way except that the descent by gravity was made with the piston in the tube so that air pressure helped retard speed. The upper terminal had sidings, with switching managed by ropes.^[29]

London and Croydon

The piston carriages were six-wheeled vans, with a driver's platform at each end, as they were double ended. The driver's position was within the carriage, not in the open. The centre axle was unsprung, and the piston assembly was directly connected to it. The driver had a vacuum gauge (a mercury manometer, connected by a metal tube to the head of the piston. Some vehicles were fitted with speedometers, an invention of Moses Ricardo. As well as a brake, the driver had a by-pass valve which admitted air to the partially exhausted traction tube ahead of the piston, reducing the tractive force exerted. This seems to have been used on the 1 in 50 descent from the flyover. The lever and valve arrangement are shown in a diagram in Samuda's Treatise.

Variable size piston

Part of Samuda's patent included the variable diameter piston, enabling the same piston carriage to negotiate route sections with different traction tube sizes. Clayton describes it: the change could be controlled by the driver while in motion; a lever operated a device rather like an umbrella at the rear of the piston head; it had hinged steel ribs. To accommodate the bracket for the piston, the traction tube slot, and therefore the top of the tube, had to be at the same level whatever the diameter of the tube, so that all of the additional space to be sealed was downwards and sideways; the "umbrella" arrangement was asymmetrical. In fact this was never used on the South Devon Railway, for the 22-inch tubes there were never opened; and the change at Forest Hill only lasted four months before the end of the atmospheric system there.^[30] A variable diameter piston was also intended to be used on the Saint-Germain railway, where a 15-inch pipe was to be used from Nanterre to Le Pecq, and then a 25-inch pipe on the three-and-a-half per cent grade up to Saint-Germain. Only the 25-inch section was completed, so a simple piston was used.^[29]

Engine house locations, South Devon Railway

• Exeter; south end of St Davids station, up side of the line
• Countess Wear; south of Turnpike bridge, at 197m 22c, down side^{[note 12]}
• Turf; south of Turf level crossing, down side
• Starcross; south of station, up side
• Dawlish; east of station, up side
• Teignmouth; adjacent to station, up side
• Summer House; at 212m 38c, down side
• Newton; east of station, down side
• Dainton; west of tunnel, down side
• Totnes; adjacent to station, up side
• Rattery; 50.43156,-3.78313; building never completed
• Torquay; 1 mile north of Torre station (the original terminal, called Torquay), up side

In the Dainton engine house, a vacuum receiver was to be installed in the inlet pipe to the pumps. This was apparently an interceptor for debris that might be ingested into the traction pipe; it had an openable door for staff to clear the debris from time to time.^[31]