APT Inter-City

The following is reproduced from a document titled "APT Inter-City".

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The Aim of the APT

The APT aims to reduce journey time to a minimum consistent with the use of existing track and signalling equipment and to provide superlative comfort.

The improved suspension, power and braking systems of APT permit speed increases over existing track and signalling of about 50%. This will reduce journey times on routes where APT is introduced to about two-thirds of 1972 times.

This approach of investment in trains rather than track is between four and ten times cheaper, depending on frequency of service and difficulty of terrain.

Wheelset dynamics

The key to improved suspension performance lies in a better understanding of wheelset dynamics.

Improved stability and guidance and reduced track loadings enable the APT to travel faster not only on the straight but also to negotiate curves up to 50% faster.

The conical profiles of wheel treads cause a rolling wheelset to steer itself back towards the track centre line if displaced sideways. When a wheelset is fitted to a vehicle through lateral and longitudinal spring and damper connections, the suspension forces are reacted at the wheel/rail interface by creep forces which modify the pure rolling motion.

Creep forces occur whenever tangential forces are transmitted across the wheel/rail contact patch. Elastic strains are present which slightly modify the rolling motion.

Up to the wheelset's critical speed, the resultant motion is stable, but above this speed, the wheelset's oscillations increase in amplitude until limited by the wheel flanges. This limited cycle is known as hunting. On curved track, additional lateral forces must be reacted by creep forces at the wheel/train interface. If the applied lateral force exceeds the available creep force, however, the wheel flange takes over.

The improved suspension design on APT allows the creep forces to be more effectively utilised and raises the wheelset's critical speed.

Traditional tread profiles are purely conical at 1 in 20 or 1 in 40 to give a sufficiently high critical speed. Such low cone angles give little guidance and the flanges are much used in curves. Also there is little conformity between new tread and rail shapes, so initial wear is rapid until a sufficiently large contact area has been achieved, by which time a hollow tread profile has resulted, with high effective cone angles and low critical speed.

The APT tread profile design is a high conformity low wear profile, hollow even when new. The lateral and longitudinal suspension stiffness and damping are chosen to ensure a high critical speed despite the conicity.

Recent measurements have revealed the vertical and lateral main-line track irregularity and roughness levels. In order to minimise the resultant forces on track and passengers of passing over current track at higher speeds, a low unsprung mass is necessary.

To achieve low unsprung mass, unconventional wheelsets are being developed for APT, some with radial resilience between axle and rim.


Because of improved guidance, maximised by the steering of the articulated layout, the APT can negotiate main line curves 50% faster than conventional trains with equal or greater safety margins.

Absence of flange contact reduces flange and track wear and improves passenger comfort.


The power required to propel a train at 250 km/h (155 mile/h) is four times that required at 160 km/h (100 mile/h). Power required to accelerate and climb hills is proportional to train mass. The 250 km/h APT, because of its light weight and streamlined construction, has only 80% of the weight and drag of a 160 km/h conventional train of the same seating capacity, despite the much greater output of its power and braking equipment, and the extra tilting and air-conditioning equipment carried to ensure passenger comfort.

The light-weight power equipment chosen is automotive type gas turbine or 25 kv electrical equipment on electrified routes.

Sixteen times more power would be required to propel the APT at 400 km/h (250 mile/h) than at 160 km/h (100 mile/h).


As with propulsion, braking power required at 250 km/h is four times that at 160 km/h for the same stopping distance. Even allowing for the APT's lighter weight, use of conventional friction brakes does not seem appropriate.

Accordingly, a light unsprung weight hydrokinetic brake is being developed which dissipates energy by heating water churned within it, as in the familiar engine test dynamometer. The heated water is transferred to the vehicle body and cooled in a radiator system.

As the effectiveness of such a brake is much reduced at low speeds, its deficiency is made up by a light duty friction brake.

Passenger comfort is maintained by a controlled application and release rate, with a maximum deceleration in a signal stop of 15%g. Electronic protection against wheel locking is provided.


Railway track is canted on curves, that is the outer rail is higher than the inner rail by up to 4 deg, so that the side force on passengers is reduced when conventional trains traverse the curves at speed.

Because the APT curves up to 50% quicker, its body tilts up to 9 deg to eliminate the side force on the passengers at all steady speeds.

The hydraulic tilt mechanism is activated by an accelerometer. This reduces the time lag compared with a passive pendular system and allows the tilt axis to be near the coach centre, minimising transient roll accelerations on the passengers and maximising use of the standard coach loading gauge.


The light-weight aluminium structure plays the major part in reducing total train weight. The structural weight per passenger is about half that for conventional trains, while still meeting BR and UIC strength and stiffness requirements.

Further structural weight reduction would greatly increase cost.

The APT articulated sets have a 3-axle power/driving car at each end with 2-axle trailer cars between.

Environmental standards of ride, noise and air conditioning will be as good at 250 km/h on the APT as on conventional trains at 160 km/h.


Careful design of nose and tail shapes and avoidance of unnecessary protuberances minimise drag and hence installed power.

The need to avoid overturning in very strong side winds sets a limit to the amount of weight reduction practicable, and careful design of the upper sides and roof shape minimises vortex generation and shedding in steady side winds.

Rapid pressure changes occur when a fast moving train passes another train or lineside object or enters and leaves a tunnel. Because the APT has a streamlined nose and reduced cross-section compared with a conventional train, these transient pressures are reduced for a given speed.

To increase passenger comfort, the air-conditioning system inlets and exhausts are automatically closed while passing through tunnels, thus isolating the coach interior from the more severe transient pressure changes

The Present

Inter-City is the brand name introduced in 1966 to identify for marketing purposes the main line passenger services of British Rail. These represent an important and growing part of British Rail's total passenger business which earns about half the total gross revenue.

Inter-City is a growing part of the passenger movement market, about 15% of all journeys over 100 miles being made by rail. After years of decline this proportion is growing - and the trend is accelerating.

In the early 60's, as dieselisation was completed on Inter-city services, fresh impetus was given to a programme to improve both speed and frequency. Today many cities on the principal routes from London have trains running at average speeds of over 70 m.p.h. with hourly frequency (and some even more frequently for much of the day).

Some reductions in service on minor routes (and particularly seasonal services) were more than offset by increases in services on the more buoyant routes. Results are well above average where important service improvements have taken place, an outstanding example being the growth of business on the electrified line from Euston to Bi rmingham, Liverpool and Manchester. Even after the initial impetus, growth has continued at a steady rate. This compares with the striking decline in airline flows between London, Liverpool and Manchester.

These results have been achieved by substantial improvements in service, largely based on research. A mathematical model has been constructed to calculate the effects of various factors, e.g. speed, frequency, road speed, price, socio-economic data etc. on passenger traffic volume on each principal route since 1964. This had demonstrated the prime importance of 'perceived journey time' which is a measure of speed and frequency combined.


Speed by itself is a crucial factor. Statistics show increases in speed and traffic volume for cities at varying mileages. A consistent pattern emerges in spite of the fact that the M1 London-Leeds motorway was completed in 1968 and there is airline connection at Newcastle and Edinburgh. It is not only a question of abstraction of traffic from other modes. Increased speeds reduce journey times to a level at which new traffic is generated.

APT will raise average speeds to nearly 100 m.p.h., while hourly services are planned for most routes including both 400-mile London-Scotland routes. More routes will have half-hourly peak frequencies. Frequency improvement is self-generating; the attractiveness of the service creating an increase in business which leads to an increase in service.


One of Rail's advantages is the location of terminals in the heart of big cities, overcoming the urban road congestion suffered by both road and airline passengers. And from the terminals of the Inter-City networks there are good interchange facilities. It is clear from Market Research that the quality of a station has a major influence on traffic growth. Modern ideas of terminal design, as demonstrated by Euston station, provide an environment and facilities which compare favourably with the better airline terminals. Shopping, office and hotel development associated with them enhance the attractions of rail travel.


Impressive improvements in rolling stock have taken place over the past few years, ranging from the introduction of bogies with superior high-speed riding qualities to electric heating, ergonomically designed seats and wide doors. Air conditioning and double glazing are provided on all new stock.

Refreshments are available on virtually all Inter-City trains through British Rail's own catering organisation, one of the largest in the country.

APT rolling stock will be built to an even higher standard of comfort than today's air-conditioned coaches. It will offer a clean, quiet, comfortable ride at even the highest speeds, and new developments in catering facilities will ensure that the traveller arrives at his destination refreshed as well as relaxed.

The aim is to offer to all travellers an unsurpassed all-round quality of transportation, which on past experience the public will be attracted to and be prepared to pay for.


Added to these features has been an aggressive sales policy. Price differentials operate between routes with the highest quality service.

Widespread use of reduced fares to stimulate off-peak travel has been encouraged and local management is in the initiative in this respect.

An extensive national advertising campaign is being carried out. Inter-City is now a household word in Great Britain and has been adopted as a brand name in English by certain foreign railways.

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Last Revised: 27.06.2022 13:45
by R G Latham
© 1998