Rail Technology Present and Future


ADVANCED PASSENGER TRAIN

The Advanced Passenger Train (APT) concept has been, since its origin in 1967, a high performance train for operation on existing tracks at speeds up to 250 km/h. The potential of APT, to substantially reduce journey times on Inter-City routes, results from its high speed capability on straight track, its ability to run through main line curves at speeds up to 50% higher than the permitted speeds for conventional vehicles, and its superior braking performance. The feasibility of such improved performance follows from advances in vehicle dynamics research and its exploitation by advanced suspension and vehicle design. A distinctive feature of APT has been the tilting of car bodies on curves by up to 9°, to maintain ride comfort for the passengers.

Since the inception of the APT Project, a comprehensive programme of research, design and development has been undertaken to prove the concept and develop the components embodied in the novel features of the train. This work has been based on an experimental train (APT-E), a rudimentary train for suspension development (APT-POP), and a series of component test programmes both in the laboratory and on the track.

The project has now progressed satisfactorily to the stage where the broad technical and commercial feasibilities of APT are established, and the train is being incorporated into the commercial strategy of the Board.

Because APT is planned to enter commercial service initially at a maximum speed of 200 km/h, the major aim of present work is to design a train which meets this specification, and which can be produced within a satisfactory time-scale with an acceptable level of technical risk. To be consistent with the longer-term commercial requirements, provision will be made within the basic train configuration for a stretch capability to higher speeds after appropriate development.

BUSINESS REQUIREMENTS

British Rail's first requirement is for a 25 kV electric passenger carrying prototype APT to run between London and Glasgow, some 400 miles. The following sets out some of the commercial factors which the train will have to satisfy on this route.

Journey Time

The minimum journey time consistent with the greatest margin of revenue over costs is the aim. Time-tabled times between 4 hours 10 minutes and 4 hours 15 minutes are required for a 200 km/h APT running between London and Glasgow with two intermediate stops.

Train Length

When journey times are reduced, more people travel. The expected increase in traffic is sufficiently large for it to be necessary to plan for the longest train that can be accommodated at station platforms enroute, and also for the most frequent service that can be accommodated on the most congested part of the line.

It may ultimately be necessary to lengthen some station platforms to accommodate 16 vehicle trains, including power cars (total length 340 m), so that the proposed train should be capable of extension to that length. Initially, however, train lengths of 12 and 14 vehicles are planned.

Train Capacity

As much as possible of the train length should be available for passenger seating, commensurate with the requirement to provide sufficient power, van space and catering facilities. The ratio of first class seats to second class seats should be between 1 to 3 and 1 to 4.

As an example of what is implied, a 14-vehicle train could have two power vehicles, two half-vehicles for van space and ten vehicles plus two half-vehicles for passengers. Allowing for catering provision, the train could seat 138 first class passengers and 476 second class passengers. This assumes 48 first class seats per coach and 72 second class seats per coach.

As a further example, a 12-vehicle train, including one power vehicle and two half-vehicles for van space, could seat 138 first class passengers and 420 second class passengers.

Finally, the extended 16-vehicle train, including two power cars and two half-vehicles for van space, could seat 186 first class passengers and 548 second class passengers.

Passenger Amenities

Features which have been agreed include two wide doors per vehicle, two re-circulating chemical toilets per vehicle, wide inter-vehicle gangways, luggage space between and under seats and in overhead racks, air conditioning, lighting, and public address systems to similar standards to the latest BR passenger vehicles, the Mark III coaches, and smoking accommodation in separate vehicles from non-smoking accommodation.

Catering

To meet catering requirements, food preparation units need to be allocated approximately ⅛ of the total passenger saloon length. For example, a first class kitchen for up to 162 meals at a time occupies the space of 30 first class seats. A second class galley for 128 passengers occupies the space of 16 second class seats; alternatively a self-service buffet for 252 passengers occupies the space of 36 second class seats.

The maximum walking distance for staff to serve, or passengers to be served, is to be two coach lengths.

Van Accommodation

There should be van accommodation for 4 Mg of parcels and heavy baggage per train.

Costs

The train should be produced at minimum capital and running costs, consistent with meeting the technical, commercial, and operating requirements.

OPERATIONAL REQUIREMENTS

In addition to the need to meet commercial criteria the trains will have to satisfy the requirements of the railway operators.

Speed

The maximum permitted speed for the introduction of APT will be 200 km/h. Operation at higher speeds, up to 250 km/h, will be permitted when it is technically and operationally acceptable and commercially required.

APT is required to stop from 200 km/h within the distances allowed for 160 km/h conventional trains. This performance demands an average deceleration of 0.09 g, allowing a 12.5% margin on stopping distance. It is generally accepted that this performance is practicable, without excessive risk of adhesion problems.

There is no requirement for displaying signal aspects in the cab at speeds up to 200 km/h.

Availability

  1. The train configuration should preferably be chosen to allow rapid uncoupling of the power car(s), and to allow the sub-division of the remainder of the train into trailer car rakes. This aids operational flexibility and minimises the value of equipment immobilised due to damage or major failure. The ease of separating power car(s) also permits the differing periodocities of maintenance between power cars and trailer cars to be accommodated.
  2. Equipment in trailer cars and power cars should be arranged in modules which are easily exchangeable in one working shift. A complete bogie or trailer car body should also be exchangeable in one shift, although the need for this should be less frequent than the need to change modules.
  3. Compliance with the requirements (i) and (ii) will minimise the need for complete spare trains and minimise the difficulty of splitting the trains for planned main works attention, for unplanned train attention and for retrieval.
  4. The train should not be so long as to require splitting for daily servicing and short examinations, but it may be split for heavier maintenance to avoid expenditure on long maintenance sheds.
  5. Component and system design should be such as to maximise train reliability and integrity.

Van Accommodation

Accommodation for parcels should be provided in few separate vans, predictably located along the train. Accommodation for staff should be adequate in size and suitably located.

Recovery

  1. To facilitate rescue after failure or assistance during planned diversions, provision should be made for towing or propelling by another APT or by a conventional locomotive. Adequate passenger comfort should be provided during periods without external power.
  2. Re-railing after minor derailments should preferably be possible with standard equipment.

Cleaning

The design of the exterior and interior and of the toilet system should be such as to facilitate rapid and inexpensive cleaning and servicing.

TRAIN PERFORMANCE REQUIREMENTS LONDON/GLASGOW

To achieve the largest reduction in journey time:

  1. The maximum speed should be as high as practicable. The initial maximum speed for APT on introduction is to be 200 km/h. The potential for reducing journey time even further by running at higher speeds, up to a maximum of 250 km/h, will thus not be exploited initially.
  2. The speed through curves should be as high as practicable. The safe working limit for wheel load transfer restricts the maximum deficiency of cant to 9°, and this determines the maximum curving speed.
  3. The installed power and consequent acceleration should be sufficient to exploit the high maximum and curving speeds, subject to adhesion and passenger comfort limitations.
  4. The deceleration should be as high as possible commensurate with available adhesion, and subject to a passenger comfort limit of 0.15 g. Operational requirements, however, limit the deceleration to 0.09 g. to remain within acceptable adhesion demands.
  5. The train mass and drag should be as low as practicable to minimise the amount of energy and power required.

Train Timing Studies

To study the effects on journey time of varying the journey performance parameters, computer simulations have been carried out for runs on the London to Glasgow route. Each run included two momentary stops, but the net times do not include station-stopped time or recovery time, which together add 20 minutes to the journey to give the time-tabled time. Thus to achieve the time-tabled time suggested in the Business Requirements, a net time between 230 and 235 minutes is necessary.

For these studies a datum train was defined which has one Bo-Bo power car and 13 trailing vehicles, each 20.85 m long, for passengers, parcels and baggage. The trailing vehicles are arranged in two articulated rakes, comprising in total four three-axle vehicles and nine two-axle vehicles. The articulated configuration was chosen as datum because it minimises train mass and drag. Thirteen trailing vehicles were chosen as a datum because this was considered to be the maximum number that can be operated by one power car having four powered axles. The train mass is 461 Mg. The power transmitted through each axle of the power car is 0.75 MW (1,000 hp), except at low speeds where the maximum tractive effort is limited to 36 kN per axle, corresponding to a maximum adhesion demand of 0.2 for a 180 kN axle load. This power (0.75 MW) was chosen because it was judged to be a reasonable practical limit for transmission by one axle.

The net journey time for this datum train, running at 200 km/h maximum speed and 9° maximum cant deficiency, on the route studied was found to be 233 min, thereby meeting the business requirement.

In the following sections, the effect on journey performance of varying the operating parameters is examined for the datum train. Journey performance is also predicted for other train configurations.

Cant Deficiency and Maximum Speed

Simulated runs were made with different power levels, at 100 km/h, 200 km/h and 250 km/h, and in each case sufficient power was provided to balance the train at 5% above the maximum speed. (The datum train balances at 210 km/h, 5% above 200 km/h, with its 3 MW at the rail). For all three speeds, cant deficiency was varied between 3° and 9° the results are shown in Figs 1 and 2. From these graphs it can be seen that a journey time of 233 minutes cannot be achieved by either:

  1. a maximum speed of 160 km/h regardless of cant deficiency, or
  2. a cant deficiency of 3° regardless of maximum speed.

At a maximum speed of 200 km/h, increasing maximum cant deficiency from 3° to 9° reduces journey time by 40 minutes to 233 minutes. It follows, therefore, that to achieve the required journey time, it is necessary to use a combination of high speed and high cant deficiency. Thus the train should incorporate a body tilting system, so that the passengers are not subjected to discomfort due to high cant deficiencies. The additional energy required to operate the tilt system is only some 600 kW for the London-Glasgow journey, compared with about 10,000 kW used for traction.

Power Available and Maximum Speed

The effect on journey time and energy consumption of increasing the number of axles in the train powered at 0.75 MW each from 3 to 10 for maximum speeds of 200, 215, 235 and 250 km/h and a maximum cant deficiency of 9° is shown in Figs 3 and 4. (The total number of vehicles in the train is assumed constant at 14 and the train mass at 461 Mg). From these Figures it can be seen that, with four powered axles, a minimum time of 233 minutes can be achieved for the two-stop journey between London and Glasgow at a maximum speed of 200 km/h. At this power, only 3 minutes can be saved by increasing maximum speed to 250 km/h. With eight powered axles, the journey time can be reduced by 13 minutes at 200 km/h and, if the maximum speed is raised to 250 km/h, a further 11 minutes can be saved.

Adhesion Requirement

On the London - Glasgow route, the distance travelled at an adhesion of more than 0.1 is less for the 14-vehicle datum train (with one power car) than it is for a 5000 hp Class 87 electric locomotive + 13 Mk II coaches over the same route. Fig 5 gives the comparison from Carlisle to Glasgow - the most arduous section of the route. This also indicates the advantage of the low mass and drag of the articulated configuration.

  1. The speed through curves should be as high as practicable. The safe working limit for wheel load transfer restricts the maximum deficiency of cant to 9°, and this determines the maximum curving speed.

Train Configuration

The datum articulated train (Train 1 Table 1†) was compared with three non-articulated trains having four-axle trailing vehicles, powered by a four-axle power car. Payload, power and trailer car length were varied so that all four trains had similar net journey times. All four trains ran at a maximum speed of 200 km/h and a maximum cant deficiency of 9° through curves.

It can be seen from Table 1† that Trains 1, 2 and 4 have the same revenue-earning capability, but to achieve the same journey time, the trains with four-axle trailer cars, Trains 2 and 4, demand more power and consume more energy. Alternatively for the same journey time and power, only 77% of the payload can be carried (Train 3) in four-axle vehicles.

Also, it is shown in Fig 6 that a train similar to Train 2 but with the same power (3 MW) and payload as Train 1, takes 12 minutes longer and consumes 1200 kWh more energy.

DYNAMIC PERFORMANCE REQUIREMENTS OF APT

Operation at high speeds, both on straight and curved track, demands a high dynamic performance of APT. This must be achieved to ensure that the train performs satisfactorily on the existing track and under the existing 25 kV overhead system.

Ride Quality

A comfortable ride is an essential requirement for a high-speed Inter-City train. There are four major aspects to be considered.

  1. Dynamic-Stability. The train must be dynamically stable (i.e. avoid hunting) over the whole speed range for the normal wheel to rail contact conditions encountered. This requirement applies not only for comfort reasons, but also for safety reasons.
  2. Random Vibration. Passengers should experience as good a ride at the maximum operating speed as the best of current vehicles at 160 km/ h, (i.e. about 0.03 g rms on good main line).
  3. Curving. Passengers should preferably not be subjected to steady lateral forces in curves. They must not, in any case, experience steady accelerations greater than the discomfort threshold of 0.07 g (4° cant deficiency). During transitions from straight to curved track, and vice versa, transient accelerations on the passengers should not exceed the tolerable level of 0.10 g.
  4. Impulsive Pressures. Aerodynamic pulses, both in tunnels and on open track, should be kept within accepted levels of passenger tolerance.

Interaction with Track

The dynamic requirements for APT are governed to a large extent by the need for a satisfactory interaction with the track. This is essential to avoid excessive damage, wear, and derailment risk. The major considerations are as follows:

  1. Vertical Forces. The dynamic forces and stresses at track features such as rail joints and crossings must be acceptable in spite of the high speeds. It is stipulated that the damaging effects to the track should not exceed those associated with the Class 55 (Deltic) type locomotive currently used on B.R. when running at 160 km/h.
  2. Lateral Forces. High speed performance on curves, with a maximum cant deficiency of 9° (see Fig 7), results in increased steady lateral forces on the track. To ensure that these are tolerable and do not cause shifting of the track, the dynamic forces generated by normal track irregularities must be minimised. As already noted dynamic instability, and the resulting large lateral forces, must be avoided.
  3. Steering. Contact between wheel flange and rail should be avoided as far as possible on main line curves to minimise lateral forces, maximise adhesion ultilisation, reduce wear and minimise derailment tendency. This requires the bogies to be self-steering, so that the wheelsets are able to align themselves radially under the action of wheel to rail interface (creep) forces.
  4. Safety. A number of safety requirements have already been stated, namely, dynamic stability, low lateral and vertical dynamic forces, and steering capability. A further requirement is the load transfer from the inner to the outer wheel (overturning tendency) when travelling at high speeds round curves should be within safe limits, i.e. should not exceed 33% under the highest normal steady force condition. This requirement limits the maximum permissible cant deficiency to about 9°.

Interaction with Overhead Electrical Equipment

To achieve a satisfactory interaction between pantograph and 25 kV overhead system, excessive loss of wire contact and excessive uplift of the pantograph head must be avoided, otherwise high wear rates due to arcing, contact-breaker operation, and impact damage to both overhead and pantograph equipment will occur.

Satisfactory operation with multiple pantographs is more difficult to achieve than with a single pantograph, particularly on some existing overhead equipments.

Implications of Dynamics on Train Configuration

Consideration of the dynamic performance requirements leads to some general conclusions concerning features which should be embodied in the APT configuration.

  1. The unsprung (wheelset) mass should be small to give good dynamic stability and minimise vertical and lateral track impact forces.
  2. The primary (bogie frame) mass should be small to promote good dynamic stability and give low lateral track impact forces.
  3. The secondary (body) mass should be large to provide a stabilising inertia for the bogies, give a comfortable ride, and reduce the overturning tendency in side winds.
  4. The axle load should be as large as practicable, consistent with the vertical force and contact stress limitations on the track. High axle load improves dynamic stability and hence ride quality, increases the resistance of the track to shifting laterally, gives good steering performance thereby minimising flange contact, reduces derailment risk, and enables high tractive effort to be generated on few axles.
  5. The vehicle mass centre should be low to minimise wheel load transfer on curves.
  6. Tilt. All vehicle bodies should be tilted by up to 9° on curves to counterbalance the lateral forces on passengers due to cant deficiency, and also to minimise wheel load transfer. The point about which bodies tilt should be as low as practicable. Pantographs must not be subjected to tilt; they must therefore be mounted on a anti-tilt platforms.
  7. Shape of Vehicle Bodies. The cross-sectional area should preferably be smaller than conventional vehicles to avoid excessive aerodynamic pressure pulses and reduce drag. The roof height should be as low as practicable to improve resistance to overturning in side winds.
    Also, a streamlined nose shape is required, again to reduce impulsive pressures and reduce drag.
  8. Number of Pantographs. Preferably only one pantograph should be used per train, to avoid the technical risks posed by multiple pantograph operation at high speeds.

PROPOSED TRAIN

The foregoing has set out the commercial, operating and equipment performance demands which the prototype APT must meet. These demands have resulted in the following proposals for the first passenger carrying train:

Configuration

The proposed train configuration comprises, one or two Bo-Bo power cars positioned between two rakes of articulated trailer cars. Each trailer rake comprises a number of two-axle intermediate cars and two three-axle end cars, one of which has a driving cab and the other has van accommodation and a motor-alternator set.

Three versions of train formation are proposed, as shown in Fig 8:

  1. a low-powered version, with one power car and up to 11 trailer cars;
  2. a high-powered version, with two pow er cars and up to 12 trailer cars;
  3. a stretched version, with two power cars and either 13 or 14 trailer cars.

The choice of version and number, type, and disposition of trailer cars depends on the commercial and operating requirements for a particular service.

It is proposed that prototype trains (APT-P/E) should adopt the high-powered train formation, with two power cars and 12 trailer cars, i.e. (2 + 12). This will ensure improved integrity and lower adhesion demands compared with the single power car version. Also, shorter journey times can be achieved and operating experience with a longer train can be gained.

For the (2 + 12) prototype train, the predicted time-tabled time for the London to Glasgow run, with two intermediate stops, is 4 hours exactly, including 10 minutes recovery time and four minutes station-stopped time. The train capacity could be 138 first class seats and 476 second class seats.

It is estimated that the production cost per second class passenger seat for the (2 + 12) train is about £1,830. Also the energy cost for the London to Glasgow journey, at a maximum speed of 200 km/h, is estimated to be £0.163 per second class passenger seat.

Current Collection

Each power car is fitted with one pantograph. Provision is incorporated for mounting a second, stand-by, pantograph to improve integrity if single-power-car operation is required. When operating with two power cars, which are always marshalled as adjacent vehicles, only one pantograph is raised and the other acts as a stand-by. Current is transmitted between power cars by a 25 kV link.

Transmission

The traction motors are body-mounted, four per power car, and each drives one of the four axles through a mechanical transmission. Power is transmitted-from each traction motor to a body-mounted transfer gearbox, through a cardan shaft to a final drive gearbox on the bogie frame, and thence via a flexible quill to an axle (See Fig 9).

Brakes

Each axle, powered and trailer, has a hydrokinetic (HK) brake. On powered axles, the brake is body-mounted and located between the traction motor output and the body-mounted gearbox input. On trailer axles the brake is mounted inside the large diameter tubular axle. (See Fig 10).

Each axle is also provided with auxiliary tread brakes, mounted on the axleboxes. These brakes progressively take over the braking duty at low speed, as the retarding torque from the HK brake decreases.

Suspension

Three types of bogie are proposed. One is a power bogie situated beneath each end of the power car. The second is an unpowered bogie situated at each end of each articulated rake. The third is an articulated bogie situated beneath the ends of adjacent trailer cars. All bogies have low wheelset and bogie frame masses.

The axle loads are designed to have good stability and self-steering properties. The articulated bogie is arranged so that it provides a course steering function, and enables articulated coaches of standard length (21 m) to be used, without violating the loading gauge on curves.

Each bogie is fitted with low frequency secondary lateral and vertical suspensions. The vertical suspension is actively levelled to compensate for changes in payload.

Hydraulic tilt jacks, activated by an electro-hydraulic servo tilt system, are incorporated into the bogies to tilt both power and trailer cars. A virtual tilt pivot is arranged to lie at about passenger's hip level so that the accommodation cross-section is maximised within the loading gauge.

Electric Traction Equipment

The electric traction equipment for one power car consists of a fixed-ratio transformer, choke, thyristor control equipment and four 750 kW traction motors. The traction motors are force ventilated and the transformer, choke and thyristors oil-cooled.

Auxiliary Power System

Auxiliary power is generated at 415 V three-phase by 250 kVA (continuous) motor-alternator sets in the van trailer cars. Each set is supplied from a 1000 V tertiary winding on the main transformer, via a thyristor converter. The maximum normal distance for distribution from a motor-alternator set is eight vehicles length.

Vehicles

The train comprises four basic types of vehicle: a cabless four-axle power car, containing power equipment only; a two-axle intermediate trailer car with passenger seating; a three-axle driving trailer car with a cab and passenger seating; and a three-axle van trailer with van accommodation, a motor-alternator set, and passenger seating. The first and second class trailer cars differ only in trim and number of seats. Additional internal layouts make provision for kitchen and galley (or buffet) catering facilities in the two-axle trailer car.

The trailer vehicle shell structures all incorporate wide vehicle-length aluminium alloy extrusions, seam-welded together.

Maintenance

Equipments subject to maintenance or repair are designed as easily changeable units or modules. These are to be changed at a depot, during a working shift, and sent away for overhaul or repair under closely controlled conditions at a central location.

PDF Figures 1 - 10.

† Tables missing from source document.

Extract from a document titled "Rail Technology Present and Future The British Contribution April - May 1974".

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