There are limits to the extent to which British Rail can improve speeds with the present range and style of traction.
Existing diesel locomotives and coaches are operating at the maximum speed possible within their design limitations.
Reduced journey times are a vital commercial need, and the choice which ultimately determined British Rail's strategy was between building a totally new railway or adapting the existing railway to give better journey times with new rolling stock.
European administrations, faced with a similar problem, decided on the Trans-Europ Express (TEE) approach which has resulted in a network of excellent but all first-class trains, available to a limited number of passengers on payment of a supplement.
But BR decided that money for improvement should not be restricted to a few crack trains, but should instead be spent in such a way as to provide improvements in speed and comfort for all passengers on all British Rail's main services.
The design requirements of both HST and APT are that they must be able to operate on the existing railway and at a cost per seat-mile about the same as conventional trains.
Production models of the HST are scheduled to enter passenger service towards the end of 1975, and will generate new Inter-City business on the non-electrified routes. As the HSTs are displaced by APTs on the principal Inter-City routes they will be transferred to improve services on other routes where, although the top speed of 125 mph may not be so important, their rapid acceleration, resulting from the high power-to-weight ratio, can significantly cut journey times. This "cascade" effect will give another boost to the average speed of all long-distance services.
Preliminary computer studies of likely costs and earnings have established that both HST and APT will be able to operate with advantage at speeds up to 125 mph as general replacements for locomotive-hauled trains. With minor modifications, existing colour-light signalling and automatic warning systems will cope safely with the new trains, and problems of compatibility with freight trains running at much lower speeds, at least during the next few years, can be resolved.
A variety of routes was examined from the point of view of the time-saving to be achieved. It was found that the electrified lines from London to the North West offered great potential for the use of APT because of their ability to take curves at high speed.
On the Western Region, however, easily-graded straight stretches favour the immediate use of HST with little preparatory work. The absence of intermediate restrictions and curves limits the further benefit to be derived from running the APT on the Bristol, South Wales routes until it can be operated at 155 mph. On the East Coast, HST will enable substantial time savings to be achieved, and the use of APT at 125 mph will bring appreciable extra benefit.
HST provides new standards of passenger comfort as well as offering substantial reductions in journey time.
The prototype comprises two streamlined diesel-electric power units, one at each end of the train, and seven of the new 75ft-long Mark III passenger coaches, including catering vehicles.
Each power car has a driving cab and is equipped with a Paxman 2,250 hp diesel engine with Brush electrical equipment, giving a total train output of 4,500 hp.
Advanced technology and design has enabled the weight of HST to be reduced to 379 tonnes, compared with 466 tonnes for trains of equivalent capacity now in use. The high power available for a lightweight train results in a rapid acceleration and a maximum service speed of 125 mph.
The bogies of the HST are fitted with disc brakes to ensure smooth and comfortable deceleration from 125 mph within the existing stopping distances for 100 mph trains. Wheel-slide protection is fitted on all axles.
The passenger coaches, a completely new design, have a range of features. Notably, they are fully air-conditioned to provide an even temperature in all seasons regardless of fluctuations outside. Air filtration keeps the interior clean. Double-glazing and improved sound insulation have reduced noise levels - previously a source of travel strain and fatigue - to provide a restful and attractive travel environment.
Air springs are used for the secondary suspension. These provide a softer ride and require less maintenance than conventional suspension systems.
All interior doors are automatic, operated by tread-mats. This will help passengers with luggage, parents with small children, and catering staff taking refreshments along the train.
Seats are of a new design, and for the first time the second-class cars have carpets.
The coaches are easily convertible to give different interior seating layouts and formations. Two basic forms have been built for the prototype train: second open (72 seats) and first open (48 seats).
This concept has been made possible by technological developments carried out by the Railways Board's Technical Centre, which occupies a site of 20 acres at Derby, and is among the world's best, if not the best, of its kind.
The APT is being developed as a high performance train capable of running at up to 155 mph on existing track. The capital cost will therefore be mainly that of the train, and no expensive modifications are necessary to the fixed installations of the railway system. The average speed between city centres will be lifted to the 100 mph range.
The APT embodies lightweight construction and refined aerodynamic shape. The important technical feature is, however, a unique suspension system which has been designed to enable the train to negotiate curves in complete safety and with no discomfort to the passengers at speeds up to 50% faster than those of conventional trains.
The essence of the APT concept is that a substantial improvement in performance can be obtained immediately with the employment of new vehicles, without doing anything to the track. When track and signalling are improved it will be possible to gain still further improvement in performance without doing anything to the vehicles. This seems to British Rail to be the right approach, both technically and economically.
An experimental APT powered by gas turbines is already testing the systems which will go into the prototype electric version. This will go into service in the late 70s, beginning on the West Coast main line between London and Glasgow.
British Rail knows that the market for such a product is a substantial one, and is confident that the launching of the APT will ensure a continuing commercial success for the rail passenger business and a continuing travel improvement for the customer.
The forerunners of the two fleets of trains designed to boost British Rail's Inter-City traffic.
On the left the prototype of the 125 mile/h (200 km/h) diesel powered High Speed Train. On the right the experimental 155 mile/h (250 km/h) Advanced Passenger Train.