Railnews November 1980

British Rail's Advanced Passenger Train - the inside story

A truly beautiful train!

CLASS 370 is a 14-car train. The formation consists of two non-driving motor cars marshalled in the centre of two identical rakes of six trailers. As passengers are not allowed to pass through the motor cars all train facilities, including catering, are duplicated in each rake.

Motor cars are continuously rated at 4000 hp giving a total of 8000 hp for traction.

Trailer cars are of aluminium construction using strips - extrusions - of aluminium up to 24 inches wide and welded together to form the body shell. Underneath is a centre keel to which the underframe equipment is fitted.

Individual coaches are of light-weight; a trailer open second, for example weighs (unloaded) only 24 tonnes, compared to 34 tonnes for a Mk III coach. Trailer cars are 21 metres long against 23 metres for Mk III vehicles yet within the 21 metres are the same number of seats as in HST coaches. Windows are double-glazed and larger than those fitted to HST trains. Air conditioning is of a special energy-saving design, using a high proportion of recycled air. Toilets are of the chemical recirculating type.

All doors are sliding "plug" type, power operated. When closed they fit flush into the door opening. Passengers can open and close doors but only when the guard has given an appropriate release on his control panel. The guard can himself open and close all doors simultaneously.

Behind the driver's cab is a diesel alternator set to provide electrical power in an emergency, or when the train is being towed in cases where the contact line is isolated for engineering work.

Marshalled next to the motor cars is a trailer brake first. This contains a first class saloon, a guard's office and luggage area with an equipment bay at the rear end. Here is to be found a motor alternator set and the rake's main air compressor.

Between the driving trailer second and the trailer brake first are four other trailers; one second class, a catering vehicle, an unclassed trailer for use as a restaurant and an open first.

Public address equipment is installed for use by the guard or chief steward. Provision is made in the trailer brake first for an invalid chair and attendant.

APT's aluminium construction gives a beautifully smooth, high quality surface finish, which is accentuated by the train's distinctive livery.

10,000hp installed power

MOTOR cars have welded steel body structures with a mechanical system of power transmission from the traction motor to the wheels.

The four 1015 hp traction motors are mounted within the bodyshell and drive through a transfer gearbox and a universally jointed "cardan" shaft to a final drive gearbox mounted on the bogie frame.

The axle passes through the centre of the final drive gears, being driven from them by a tubular "quill" shaft. Thus the axle is free to move slightly, both vertically and horizontally.

This arrangement reduces the unsprung weight of the wheels and axles.

Power for traction is taken from the contact wire at 25000 volts AC by a roof mounted pantograph. From this the supply is led through a circuit breaker to the main transformer which reduces the voltage.

Between the traction motors and the transformer is an electronic assembly called a thyristor converter which rectifies the transformer output from alternating current (AC) to direct current (DC) for the DC traction motors. At the same time, the thyristor convertors vary the voltage in step with the demand for power called for by the driver's power controller. Increasing current increases the power output and vice-versa.

The driver can select a total of seven power levels or notches on his power controller, plus an additional boost notch to obtain extra power under certain conditions.

When two motor coaches are in use only one pantograph is raised: a roof-mounted link couples the two cars electrically. Auxiliary electrical supplies - for air conditioning, tilt pumps etc. - are derived from a motor alternator set in the trailer brake first. This equipment is fed from a separate output in the transformer circuits.

An APT's motor car is a truly remarkable vehicle. In a car weighing only 68 tons is installed power equipment of 5000hp - 4000 for traction plus 1000 for auxiliaries.


APT uses three types of bogies. An end trailer bogie (BT12) is used under driving trailers and trailer brake first vehicles, with an articulated bogie (BT11) to link intermediate trailer coaches.

Coil springs support the bogie frames, the coach bodies being suspended on air suspension units. These air springs, designed to keep the floors of individual vehicles level and to reduce noise transmission are located on the extreme ends of BT11 bogies and on the centre line of BT12 bogie frames.

Motor cars use a heavier bogie (BP17) but with a nest of coil springs for body support. All three bogies are of steel construction with hydraulic dampers.


APT's nose end has a streamlined profile. Under the reinforced plastic nose cone is a set of retractable buffers, a coupling hook and a drophead buckeye coupler, together with brake and main reservoir air connections and an electrical jumper cable, all for emergency use. Access is obtained by lifting the nose cone.


THE braking system installed on the APT, known as hydrokinetic, or HK for short, is a unique system using a water/glycol mixture to create a braking effect on the axles. APT's particular braking problem was to bring the train to a halt from its highest speed within the same distance as conventional 100 mph trains so as to allow the existing signal spacing to remain.

The HK brake uses a relatively simple principle. A water reservoir is connected by flexible pipes and a torque reaction tube to a chamber inside the hollow axles. Inside the chamber is a stationary "stator" in the form of a disc with cup-shaped recesses arranged on its surface. Linked to the hollow axle - and the wheels - is a rotating rotor with a shape and form similar to the stator. The rotor and the stator run side by side, with cups facing each other.

During braking, a controlled air pressure is introduced above the water mixture in the reservoir which forces the water into the chamber where it is picked up by the cups in the rotor and impelled against the stationary cups on the surface of the stator. This action causes the rotor - and the wheels - to slow down. The water follows a circular path inside the cups and in doing so generates heat in the water. Rotation of the rotor pumps out the water to the water reservoir where it is cooled by a cooling system.

Seven brake steps are provided plus an emergency stop. Movement of the driver's brake controller is transmitted electronically along the train.


ADVANCES in vehicle suspension design can make higher speeds possible on curved sections, but not without considerable discomfort to passengers - unless the vehicle itself is allowed to tilt. Tilting the coach body can minimise lateral acceleration, maintain passenger comfort and allow the train to negotiate curved track sections at higher speeds than would be permitted by conventional non-tilting trains.

APT's tilting bodies are suspended on an air suspension unit carried by a swinging "bolster". As each vehicle enters a curve the tilt system measures automatically the amount of body tilt necessary to minimise side thrust, so as to maintain comfort levels. This measurement is conveyed to valves between a hydraulic (oil) pump and tilt jacks located on the bogies.

The valve opens moving the jack until the required amount of tilt is achieved and afterwards the valve closes, locking the tilt jacks in position. Once clear of the curve the tilt jacks are unlocked, thus restoring the vehicle body to an upright position. Up to nine degrees of tilt is possible.

The power car tilts round curves in the same way as passenger vehicles. The pantograph, however, is maintained in contact with the overhead wire by a tilt compensation linkage.

APT's light weight, coupled with its ability - because of tilt - to run through curves so much faster means that less energy is wasted. In fact, an APT uses one-third less energy at 125mph than a diesel HST at the same speed.


Running cost per seat/kilometer for APT is 0.195 pence compared with HST's 0.307 pence.


APT broke the previous highest speed record on BR in December 1979 when it reached 160mph during a test run on the Carlisle-Glasgow route.

RAILNEWS November 1980 Special Souvenir Extra
© RAILNEWS November 1980.
Drawings by DAVID GIBBONS.

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Last Revised: 18.01.2019 13:30
by R G Latham
© 1998