Advanced Passenger Train
The Advanced Passenger Train (APT) was an experimental tilting high speed train developed by British Rail during the 1970s and early 1980s, for use on the West Coast Main Line, which contained a lot of curves. Notable among numerous technical advancements was the active tilting system, which the APT pioneered and has since appeared on other designs around the world. Other features of the APT such as the hydro-kinetic braking, used to stop the train within existing separations, have not been adopted. The experimental version APT-E achieved a new British railway speed record when on 10 August 1975 it hit 152.3 mph (245.1 km/h).
The introduction into service of the Advanced Passenger Train was to be a three-stage project. Phase 1, the development of an experimental APT, the APT-E, was completed. Phase 2, the introduction of three prototype trains, known as the APT-P, into revenue service on the Glasgow-London route, enjoyed only limited service due to bad publicity. Phase 3, the introduction of the Squadron fleet designated APT-S, did not occur.
The APT suffered from delays between the APT-E and APT-P, and then a lengthy series of in-service problems when the APT-P started testing. As these delays grew, the programme was dismissed as a white elephant and every minor problem was heavily covered in the press. It gathered a reputation that led to politicians wishing to distance themselves from the fray. Further funding for service entry became increasingly difficult to find, and the project eventually ground to a halt.
In the mid to late 20th century, British Rail express services compared unfavourably with France's TGV and Japan's Shinkansen. Experience with increasing the speed of passenger services had shown that reduced journey times could produce a significant increase in passenger numbers. The East Coast Main Line was largely straight and suited to high speeds, but other lines, specifically the West Coast Main Line (WCML) from London to Glasgow, were not straight enough to support high speeds with conventional equipment. Lateral forces would be just too high around curves; passengers would not be able to stand upright easily, and things would move on tables. Because slower trains also use the same tracks, superelevation (banking or "canting" of the track around curves) could only be utilized to enable speeds up to 125 mph (201 km/h).
In order to permit a top speed of 155 mph (249 km/h), and thereby cut journey times, British Rail's engineers at the Derby Research Division developed an advanced active tilting technology, using hydraulic rams controlled by spirit level sensors to tilt the passenger cars into the curves so that no lateral forces would be felt.
British Rail deliberately commissioned two young, bright, inventive engineers with no experience in trains or railways to head the development of the project. One had worked in missiles and the other for the National Coal Board. They worked in isolation to the in-house British Rail engineering team. The reason for total outsiders to develop the trains was that they would have no preconceived rail engineering prejudices and would approach the problem with a fresh open mind. These fresh minds brought to trains methods and technology from other advanced fields. They brought science into British railways, an industry that from its quantum leap in innovative technology in the Victorian times, had progressed at a slow pace since then in comparison to other countries.
Not only was the train designed to tilt but it was also articulated and had hydrokinetic (water turbine) brakes. The latter feature is often overlooked but was in fact just as significant as the tilting concept, because it enabled the train to stop within the existing signal spacings. During extensive testing on the West Coast Main Line, the hydrokinetic brakes repeatedly achieved their predicted performance. The fault in the system was that the hydrokinetic braking forces fell away rapidly as speed fell below 25 mph. At that speed the system was designed to apply hydraulic pressure to conventional friction brakes on the axles of each bogie on the train. The train manufacturer produced these bogies in bulk long before the train vehicles were ready. Their friction brakes were tested independently with water before they were stored but the storage period was so long that the internal coating on the hydraulic cylinders corroded. When the trains were tested in service, this corrosion caused a loss of brake pressure on every axle. On test, this caused the train to take nearly as long to slow from 25 mph to a standstill as it did to slow from 125 mph to 25 mph. During commissioning, because of this and other development issues, every axle on the trains was modified and exchanged.
The shortcomings of using engineers with no experience of trains was exposed when the APT-P began commissioning tests. The stainless steel welds cracked on the hoses carrying fluid to the hydrokinetic brakes on the axles. Although this was rapidly solved, the air systems on the trains were not designed to deal with the water that compressed air contains. On conventional trains, vehicles were designed such that air pipes sloped towards drain valves and compressor air receivers were fitted with automatic drain valves. This reduced the water in the system and ensured that it was not trapped in pipework or critical equipment. On the APT-P, air pipe runs were tortuous, accidentally designing in multiple trap points for water. The results were first experienced during freezing conditions during West Coast Main Line commissioning, operating from Shields Depot, Glasgow. Air-powered door opening equipment froze and it was clear that the air controls on the hydrokinetic braking system would be compromised. Although the commissioning team engineers sought and found a Westinghouse designed solution that would eliminate the water being produced by the compressors, the design team would not accept the solution, stating that the problem would not occur with a full train formation, as opposed to the shorter formation used in commissioning. The seed was sown for the disastrous demonstration run from Glasgow, bearing national journalists, when the train failed shortly after leaving Glasgow due to a frozen brake system. The project never recovered.
It was only discovered at the APT-P commissioning stage that parts of the West Coast Main Line had been built in such a way that, if two APT-P trains with their tilt systems failed and the carriages stuck in the inward tilted position met, they would strike one another. The civil engineers had built the railway with dynamic envelopes too small for the ATP. The effect was not seen with conventional trains since, without tilt, their movements stayed well within the dynamic envelope.
Some of the senior managers in British Rail at the time were unwilling to commit to a single project, and so initiated a parallel project to design a train based on conventional technology as a stopgap. This was the High Speed Train (HST), which was also marketed under the InterCity 125 name. The same senior managers withheld experienced engineering resources from the APT project, using them instead to press ahead as swiftly as possible with what they saw as a conventional rival to APT. The HST was a successful design, and is still in use 30 years later.
In 1972, the APT-E, a gas turbine-powered experimental testbed, was constructed. This was only four cars in length; two power cars, one at each end and two 'passenger' cars full of instrumentation. The experimental train APT-E having proved the concept, British Rail moved to build three prototype Class 370 APT-P trains. Gas turbines had been chosen for their light weight compared to diesel engines but Leyland had ceased production and development and no other was suitable. Thus the new APT-P and APT-S trains were to be electrically powered and so restricted to electrified track. The hydrokinetic brake system was successful and reliable on the APT-E but for the APT-P design, it was decided to abandon the oil-filled hydraulic system and design an all-new system filled with water glycol solution, to reduce costs. The exhaustion of the corrosion inhibitor in the static hydraulic lines of parts of the water glycol system was to cause multiple problems with the brakes during commissioning.
The APT-P trains were designed as two half-trains with twin power cars in the middle, sharing one pantograph. There was a passage through the power cars but it was noisy, cramped and not permitted for passengers. Therefore, each end of the train had to duplicate facilities. There were a number of reasons for this design compromise. Two power cars were necessary to maintain the design speeds over the northern banks with 12 coaches. Normally these would be situated at the front and rear of the train (as with the HST and TGV etc.) but, due to the design of the overhead line, a "wave" was set up in it by the front pantograph, thus causing problems for current collection from the rear unit. The obvious answer was an on-board 25 kV "roof-line" link to the rear power car but this was considered infeasible at the time. The final option was to put both power cars at one end of the train but, at the high speeds (and with the tilt feature), concerns were raised over excessive buckling forces when the train was being propelled. The eventual decision to use two non-articulated power cars in the centre of the train allowed one pantograph to be used with a 25 kV link between the two power cars. Both power cars were equipped with pantographs, with one lowered in service. Power was supplied through ASEA thyristor equipment, supplying four 1 MW DC traction motors mounted in each power car. The traction motors transmitted their power through internal gearboxes, cardan shafts and quill final drives, minimising the unsprung weight on the axles.
Although all auxiliary equipment such as lighting, air conditioning and air compressors was powered by motor alternators, from the 25 kV overhead line, it was recognised that if there were to be an overhead line failure conditions in the passenger vehicles would quickly become unsafe and unbearable. Each driving van trailer i.e. the leading and trailing vehicles, was equipped with a diesel-alternator generator capable of supplying the minimum requirement of auxiliary power. The diesel-alternators were started using air motors powered from the train's air system, since APT carried few batteries.
The APT was designed for faster running than existing trains, and over the same track. The higher speed limits were provided to the driver by way of a transponder-based cab display called "C-APT". A radio beam from the train caused a track-mounted transponder to return the current line speed for an APT, which would then be displayed in a cab instrument. These sealed, unpowered transponders were placed at intervals of no more than 1 km. Approaching speed restrictions were provided at the appropriate distance, along with an audible alert; failure to acknowledge these alerts would result in an automatic brake application. C-APT was driven by a redundant onboard computer system using Intel 4004 microprocessors. The track units were essentially the same as the modern French Balise beacons.
Political and managerial pressure to show results led to the three APT-P trains being launched in 1981 when, in hindsight, they were not ready for service; many technical problems persisted and reliability was not high. Predictably, the train suffered highly visible problems. Two APT-Ps were intended to be available for service at any given time, with the third out of service for overhaul and maintenance. The APT was often jokingly referred to by passengers as the 'Accident Prone Train' because of this.
Members of the press riding the first demonstration train apparently reported high levels of motion sickness and this caused considerable bad publicity; though it has been suggested by some, including APT designer Prof. Alan Wickens, that the 'motion sickness' suffered by the press may have had more to do with them over-indulging in BR's "liquid hospitality". None of the other passengers on this demonstration run or subsequent runs noticed the problem. The commissioning team, however, was familiar with the feeling of motion sickness. The tilt sensors on each vehicle, which determined when and by how much a vehicle should tilt on curves, detected the need for tilting too late, so that a smooth transition into tilt, unnoticed by passengers, didn't occur. The tilt system didn't "lean into the curve" as it approached. Effectively passengers experienced the curve, then the tilt cut in, lagging the curve. The eyes could see turning but the body did not feel it in synchronisation; moving the tilt sensor control to the preceding vehicle and reducing the tilt by a few degrees so that the curves could be felt cured this.
On one test run with the press, certain units of the train 'stuck' tilted to one side for parts of the journey.
The first "public" APT-P run on 7 December 1981, from Glasgow Central to London Euston, was successful. Even so, British Rail played safe by running a scheduled service out of Glasgow some 15 minutes later. Some APT-P cars suffered tilt failures during the return trip out of London and this was widely publicised by the media. The extremely cold weather also caused problems with the brakes freezing. The trains were withdrawn from revenue service four days later. This highly visible failure was eventually to prove terminal for the project.
The APT-P trains were quietly reintroduced into service in mid-1984 and operated regularly, the problems having been apparently corrected but the political and managerial will to continue the project and build the projected APT-S production vehicles had evaporated.
One APT-P set was kept at Glasgow Shields Depot and found use once or twice as an "EMU" to take journalists from Glasgow Central to Anderston railway station and back, for the Scottish Exhibition and Conference Centre. A second APT-P was stored in a siding behind Crewe Works. The "Glasgow" APT-P and the third APT-P were scrapped very quietly without publicity.
Williams notes that work continued on a new variant, the APT-U, and that the project was later retitled InterCity 225, perhaps to distance it from the bad publicity surrounding the APT-P. The Mark 4 coach design that was introduced as part of the new IC225 sets for the East Coast Main Line electrification is a direct descendant of the APT-U, and the coach was designed for the retrofitting of the tilt mechanism, although this was never implemented. The Class 91 locomotives that power the IC225s also take many features from the APT-P power cars, including body- rather than bogie-mounted traction motors to reduce unsprung load, and having the transformer below rather than on top of the underframe to reduce the centre of gravity. Unlike the APT-P power cars, though, they were never intended to tilt.
The APT-E unit is now owned by the National Railway Museum and is on display at their Locomotion museum at Shildon in County Durham. Also at Locomotion is an APT-P power car, number 49006, which is due to undergo restoration work after having been stored outdoors at York for many years. The second APT-P unit is now on display at Crewe Heritage Centre and can be seen from trains passing on the adjacent WCML. On 19 October and 20 October 2013 the APT-P unit at Crewe was tilted for the first time since the 1980s.
While tilting trains had been in development in other countries for some years, and even seen service, so-called 'pendular tilt' had not been particularly satisfactory. The designs of the APT 'powered tilt' carriages were sold to Fiat Ferroviaria, which exploited the technology in the design of the second generation of the functionally similar Pendolino trains, coupled with an in-house electronic control system (first generation systems had been in service since 1976). Italian Pendolino systems incorporating original APT technology have since been sold internationally, including the British Class 390 Pendolino introduced on the West Coast Main Line from late 2003 onwards, culminating in September 2004 with the introduction of a full passenger timetable, with tilting enabled from Manchester and Birmingham to London Euston.
The introduction of the Squadron fleet designated APT-S did not occur as had been originally envisaged. The APT project succumbed to an insufficient political will in the United Kingdom to persist in solving the teething difficulties experienced with the many immature technologies necessary for a ground breaking project of this nature. The decision not to proceed was made against a backdrop of negative public perceptions shaped by media coverage of the time. The APT is acknowledged as a milestone in the development of the current generation of tilting high speed trains. 25 years later on an upgraded infrastructure the Class 390 Pendolinos now match the APT's scheduled timings. The London to Glasgow route by APT (1980/81 timetable) was 4hrs 10min, the same time as the fastest Pendolino timing (December 2008 timetable). Recently, on a one off non-stop run for charity, a Pendolino completed the Glasgow to London journey in 3hrs 55min, whereas the APT completed the opposite London to Glasgow journey in 3hrs 52min in 1984.
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|Wikimedia Commons has media related to Advanced Passenger Train.|
- APT-E at Old Dalby
- APT-P high speed pantograph tests on WCML
- Wickens, A., (1988) APT - With Hindsight Newsletter of the Friends of the National Railway Museum, No.84, Summer 1988
- Advanced Passenger Train Info Site
- Advanced Passenger Train Restoration Site