The Advanced Passenger Train project is designed to provide a cost effective solution to the problem of providing fast inter-urban transport on existing tracks, in both financial and energy consumption terms.
APTs have developed from a programme of research into the dynamics of rail vehicles - passenger and freight - which was started in 1964 at the then newly established Railway Technical Centre, Derby.
The cost of the research and development phase of the programme, including building and running the experimental train APT-E has been £10m.
During the highly successful development phase, the experimental train not only became the fastest train ever to run in Britain, running at 152 mph (243 km/h) on 10 August 1975, but even more significantly, two months later it covered the 99 miles (159 km) between London and Leicester in 58½ minutes. It was this demonstration which showed just how effectively APTs can improve on the performance of present day trains. The fastest scheduled Inter-City trains are timed to cover the London-Leicester distance in 1 hour 24 minutes, an average speed of 70.7 mph (112 km/h), APT-E cut the journey time by nearly one-third, averaging just over 100 mph (160 km/h).
The project has now moved from the experimental stage. In October 1974, with Government approval, British Railways Board authorised the building of three prototype electrically propelled passenger carrying APTs at a cost of £9.9m.
The total cost of the APT prototype programme is about £25m, including the three trains, production line and other development costs which will not recur and major items such as maintenance depots which will be required for subsequent trains. £11.6m has been loaned to the project by the European Investment Bank.
When APT-Ps enter experimental commercial service, the maximum speed will be limited to 125 mph (200 km/h) giving a possible journey time of 3 hrs 57 mins from London to Glasgow with one intermediate stop. This compares with the best present day timing for the 401 mile (640 km) journey of 5 hours. Each train will have accommodation for 592 passengers.
Test running will start in the second half of 1977. The trains are being built by British Rail Engineering Ltd at their works in Derby.
A major attraction to Inter-City rail passengers is shorter journey times. Improved services on BR's West Coast main line following electrification from London to Manchester/Liverpool have provided considerable evidence of this, so has experience abroad, such as on the Japanese Tokaido line. Comfort, reliability and safety, and price and frequency of service are also important.
Shorter journey times can only be achieved through investment in new trains, new infrastructure, or both. BR's extensive railway network is the legacy of massive Victorian investment. The construction of new tracks or major new alignments would require further large investment and would absorb capital which might otherwise be available for trains. From the national viewpoint, therefore, it is preferable to exploit the existing assets to their full potential and concentrate effort on designing high-performance trains for existing tracks.
The obvious way of reducing journey time is to install more power and operate at a higher maximum speed. However, roughly 50% of BR's major routes is made up of curves, and of these about 50% are relatively sharp at between ⅓ mile (0.5 km) and 1¼ miles (2 km) radius. Thus, average speeds of conventional trains tend to be determined largely by speed restrictions due to curves, and only modest gains can be made for a speed capability above 100 mph (160 km/h) on all except the straightest routes.
The major objective of APT therefore is to significantly raise operating speeds round curves, so that a high maximum speed results also in a high average speed.
Speed restrictions on curves are currently applied not for reasons of safety, but to limit the discomfort of passengers when subjected to centrifugal forces. The permitted speed is governed by the track radius and cant angle in conjunction with a limiting value of 0.7 m/s² for the net lateral acceleration. This corresponds to a maximum deficiency of about 4° in the cant angle.
APT aims to increase cant deficiency to 9° equivalent to a net lateral acceleration of 1.5 m/s². This raises the maximum speed through curves by typically 20 to 40%, the exact speed ratio being proportional to the square root of the cant deficiency plus track cant angle.
Operation at these higher speeds would be intolerable for passengers in a conventional train. However, by tilting the APT vehicle bodies inwards by up to 9°, comfort is not only restored, but enhanced by fully compensating for cant deficiency.
For greatest impact on journey time, both maximum speed and cant deficiency must be raised. Accordingly, APT performance boundaries are specified by 150 mph (250 km/h) and 9° cant deficiency.
At high speeds, the major source of train resistance is aerodynamic drag, which increases as speed-squared. Consequently, the energy and power used in traction tend to rise rapidly as operating speeds are increased.
Although train weight has only a small effect on energy consumption at constant speed on level track, its effects are important when climbing hills and during accelerating and braking. Train weight influences particularly the number of powered axles, and hence the amount of power equipment required for operation at acceptable adhesion levels.
For economic performance, therefore, a streamlined low-drag profile has been adopted for APTs together with lightweight construction. Since the effect of acceleration rate on Inter-City journey times is of secondary importance, only sufficient power is installed to give a train balancing speed on level track about 5% above the maximum operating speed.
The low aerodynamic drag of APT results from its nose and tail shape, its reduced cross-section, including lower roof height, and its general surface smoothness. Drag measurements with APT-E have confirmed that at 100 mph (160 km/h) APT consumes only two-thirds the energy of a present-day train. Put another way, APT uses the same energy at 125 mph (200 km/h) as existing trains do at 100 mph (160 km/h).
The broad objectives set for APTs in relation to the performance of the then existing trains, were that they should have a maximum speed 50% higher, the ability to negotiate curves at up to 40% faster, to run on existing track with existing signalling, to maintain the standards of passenger comfort at the higher speeds, to be efficient in energy consumption, to generate low community noise, to maintain existing levels of track maintenance, and to achieve a similar cost per seat-kilometre. In general, the aim was for APT to be as technically adequate at 150 mph (250 km/h) as the existing trains were at 100 mph (160 km/h).
The first three passenger carrying APTs will be provided with electric traction using the 25 kV 50 Hz overhead supply system.
This conforms with the long-term strategy for British Rail's Inter-City business which calls for the benefits of APT performance to be applied first of all to the West Coast main line between London and Glasgow.
An APT for non-electrified routes is now likely to be diesel-powered, since the successful production of a suitable regenerative gas turbine cannot yet be foreseen.
The 25 kV electric APT comprises two rakes of articulated trailer cars between which are positioned one or two power cars. Each trailer rake consists of a number of two-axle intermediate cars and two three-axle end cars. Each power car has four axles, each with a continuous tractive power rating of 750 kW. Power cars and trailer rakes are easily uncoupled from each other in order to satisfy operating and maintenance requirements.
The train can be formed into three alternative versions. The choice of versions, and of number, type and disposition of trailer cars, depends on the commercial and operating requirements for a particular service. The 125mph (200 km/h) (1+11) low-powered version, with 11 trailer cars, represents the longest train that can be hauled by a single power car up the 1.5% gradients on the West Coast main line. The 150 mph (200 km/h) (2+12) high powered version, with 14 vehicles total, is the longest train which can be accommodated within the existing platform lengths. The 150 mph (250 km/h) (2+14) stretched versions is the longest formation envisaged. Its application would require some investment to lengthen platforms. However, it has the advantages of a high capacity and high speed capability at a similar cost per seat-kilometre to the (1+11) version.
The (2+12) version has been adopted for the prototype train APT-P so that the full design performance can be proven. By re-marshalling vehicles the other versions can also be tested. When APT-P enters experimental commercial service, the maximum speed will be limited to 125 mph (200 km/h) giving a possible journey time of 3 hrs 57 mins from London to Glasgow with one intermediate stop. This compares with the best present-day timing of 5 hrs. It is probable that fleet service will be initiated with the (1+11) version at 125 mph (200 km/h), retaining the option for two-power-car versions and higher speeds when commercially and operationally justified.
The train configuration, with power cars positioned in the middle, has been adopted for a number of reasons. Firstly, experiments have shown that the collection of current with more than one pantograph is likely to be unsatisfactory at 125 mph (200 km/h) with some existing overhead equipment. There are, therefore advantages in locating two power cars adjacent to each other, with current being collected by a single pantograph and transmitted locally between power cars by a 25 kV link. Secondly, excessive train buckling forces would be generated under propelling conditions if two power cars were to be positioned at one end of the train. Thirdly, by avoiding the need to fit a cab and heavy buffing and drawgear to the power car, it has proved feasible to install, within the severe vehicle mass limit of 69 Mg, sufficient thyristor-controlled traction and auxiliary power equipment to give a total power output of 4 MW. Consequently the (1+11) version is able to achieve 125 mph (200 km/h) with only its one power car. Fourthly, for operational flexibility, it is advantageous to have two relatively short equal-length rakes of articulated trailers rather than one long rake. The use of a central power car permits this with the minimum addition to train mass and with the minimum number of vehicle types.
Communication between the two trailer rakes will normally be available to staff only, via a corridor through the power car. Passengers will be permitted access only in emergencies and when escorted by a member of staff. To minimise this disadvantage, each rake of trailer cars will be self-contained, incorporating both first-class and second-class accommodation and catering facilities. First-class accommodation will be in the middle of the train and second-class towards the ends, the division being marked by the intermediate catering car. The catering unit will provide full meals for first-class passengers in their seats and a buffet service for all passengers.
Auxiliary power generation equipment for the train will be carried at the two ends of each trailer rake, thereby improving overall weight distribution. A 400 kW motor-alternator set will be mounted in each trailer car next to the power car. This vehicle will also accommodate a parcels van and guard's compartment. A 200 kW diesel-alternator set will be mounted in each driving trailer car, behind the cab. This set will provide power during emergencies and when the train is towed on non-electrified lines during planned diversions. To facilitate towing, the cab, which will be similar to that of the High Speed Trains in internal layout, will carry a nose section which can be hinged upwards to reveal conventional buffers and drawgear.
So that higher curving speeds can be safely exploited, a display in the cab advises the driver of the permitted maximum speed for APT at any instant. The system is based on transponders (passive micro-electronic receiver and transmitter devices) which are mounted on the track at intervals. These are interrogated by the train as it passes over them, causing the transponder to transmit coded speed limit information. A driver can thus drive APT using the speed limit display in conjunction with his standard route knowledge only. Advance warnings of permanent speed restrictions are given to indicate when braking should be initiated. Whenever the display is blanked out, the driver must obey standard speed limits. These apply, for instance, at the approaches to terminal stations.
Unsprung mass and bogie weight have been minimised by using flexible axle drive quills and by mounting the separately-excited traction motors off the bogies and inside the power car body. Power is controlled by thyristor convertors and is transmitted independently to each driven axle via a bodymounted gearbox, cardan shaft, and lightweight final-drive reduction gearbox. The final-drive gearbox is fully suspended on the bogie frame. Each power car is carried on two two-axled bogies, each motor driving an axle.
An advanced design of pantograph, with three-stage suspension, is being developed for APT-P. This is mounted on an anti-tilt mechanism which maintains the pantograph head over the track centre-line. Although the pantograph is restrained to the bogie in roll, it is otherwise fully suspended with the vehicle body.
The braking system for APT is designed to stop the train from 150 mph (250 km/h) within the existing signalling distances for 100 mph (160 km/h) trains, including a 12.5% margin.
Hydrokinetic brakes were chosen to meet the arduous braking duty because they were capable of dealing with the very high levels of energy dissipation (35 MJ per trailer axle) and power dissipation (1.5 MW peak per trailer axle), whilst complying with the limitations on unsprung mass and bogie mass. The combination of high energy, high power, and low unsprung mass makes conventional friction brakes unattractive.
The hydrokinetic brake develops a torque, when filled with fluid, in the same way as the familiar engine dynamometer. The brake is mounted inside a hollow axle on all except driven axles, and the energy of braking is converted into heat as angular momentum is successively generated and destroyed within the fluid. The torque produced is proportional to the pressure rise through the brake; regulation of this pressure thus provides a simple means of controlling torque. The fluid (water-glycol) is pumped by the brake itself to a body-mounted reservoir, and the heat is subsequently dissipated in fan-cooled radiators. The continuous power rating for the brake on APT-P is 6 W/kg of train mass.
Because the hydrokinetic brake loses its effectiveness at low speeds, an auxiliary light-duty hydraulic friction brake acting on the wheel treads is also provided. This brake is automatically blended in at speeds below 50 mph (80 km/h).
Each powered axle on APT-P is hydrokinetically braked, the brake being fitted to the body-mounted gearbox in the mechanical drive to the axle.
Lightweight vehicles are required not only for low total train mass, but also for acceptable axle loads, especially on articulated axles. To meet the mass targets, APT trailer car bodyshells are constructed in aluminium alloy, giving a 40% weight saving over a conventional steel coach. Lightweight steel construction has been adopted for power car bodyshells.
The vehicle structures are designed to meet the International loading specifications for main-line coaches, including the 2 MN proof buffing load. They are also designed to be stiff enough to avoid strong vibration coupling between body and bogie frequencies. The dominant design criterion tends to be stiffness rather than strength, as high flexural natural frequencies are essential for good ride quality at high speed. APT body structures are therefore designed for fundamental lateral and vertical bending frequencies of about 15 Hz when fully equipped.
The form of construction adopted for APT-P is based on extensive use of wide commercial-grade aluminium extrusions running the full length of the vehicle and making up the outer profile. These are seam-welded together automatically and the completed bodyshell has a mass of 4.8 Mg.
The APT-P power car is a steel semi-monocoque structure with deep side skirts. High stiffness is attained with an efficient distribution of mass, resulting in a bodyshell mass of only 12 Mg.
To avoid vehicle bending frequencies being severely depressed, with subsequent penalties on structural vibration levels, a form of articulation which allows relative movements between vehicle ends has been adopted. This has led to a simple and light articulated bogie design.
The tilting of vehicle bodies by up to 9° has been a major influence on APT design. Tilt produces severe space limitations, controls many suspension parameters, and generates some of the dominant structural loading cases. The requirements for tilt were therefore fully integrated into the train design from the outset.
Because of the restricted space envelope prescribed by the BR loading gauge, the axis about which each vehicle must tilt is governed largely by the requirements for acceptable body width and profile.
The existing transition curves between straight and constant-radius track are relatively short for high, speed operation, resulting in a demand for high tilt response rates up to 5°/s Such rates cannot be reached by a passive pendular type of tilt suspension, especially as the tilt centre nearly coincides with the body mass centre. Consequently, each vehicle is tilted independently by an active electro-hydraulic servo-system which responds to sensors measuring the lateral accelerations experienced by the passengers. The tilt system design, permits the vehicle body to adopt an upright position in the event of hydraulic system failure.
Passenger doors on APT-P are of the power operated sliding plug type with a retractable step. By having only two wide doors per vehicle, located in diagonally opposite corners, it has been possible within a total vehicle length of 66½ ft (21 m) to accommodate 72 second-class or 47 first-class seats at standard pitch.
Internal fittings such as seats, luggage racks and trim panels are modular and easily replaceable. Similarly, air conditioning, tilt control and brake control units are installed in the vehicle underbelly as pre-commissioned, readily removable packs.
Substantial weight savings have been achieved by developing a chemical toilet, lightweight seats (12 kg/second-class seat), and a low-energy air conditioning system. This system uses a low fresh air charge and a high (80%) re-circulatory flow, de-odorised by carbon filters. The small intake and exhaust areas are sealed on entry to tunnels to protect passengers from transient pressures.
The passenger noise environment inside APT is required to be not more than 68 dBA and 76 dBB.
The APT has been designed for a maximum speed of 150 mph (250 km/h) with relatively minor modifications it could run at 190 mph (300 km/h). The maximum worthwhile speed, however, is determined not simply by technical capability but by commercial and social factors. Higher speeds are worthwhile until revenue rises more slowly than costs, or until the economic, social and environmental advantages to the nation grow more slowly than the disadvantages.