Advanced Passenger Train - Background

Background

Main articles: British Rail APT-E and British Rail Class 370 See also: InterCity 125 and British Rail Classes 253, 254 and 255

In the mid to late 20th century, British Rail express services compared unfavourably with France's TGV and Japan's Shinkansen. Experience with High Speed Trains on the East Coast Main Line from London to Edinburgh had shown that reduced journey times could produce a significant increase in passenger numbers, but that line was largely straight and suited to high speeds. Other lines, such as 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 utilised 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. Numerous engineers involved in the project had an aeroengineering background. 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. The fact that under operating conditions it failed to do so was one of the main factors in the train being withdrawn.

Some of the senior managers in British Rail at the time were unwilling to put all their eggs in one basket, 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 HST went on to become one of the most successful designs ever 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 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 normally 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 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.