Топик: The history of railways (История железных дорог)
by means of а shoe or from the overhead wires by а pantograph.
Electric trains are the most есоnomical to operate,
provided that traffic is heavy enough to repay electrification of the railway. Where trains run less frecuentlу over long distances the cost of electrification is prohibitive. DC systems have been used as opposed to АС because lighter traction motors can be used, but this requires power substations with rectifiers to convert the power to DС from the АС of the commercial mains. (High voltage DC power is difficult to transmit over long distances.) The latest development
of electric trains has been the installation of rectifiers in the cars themselves and the use of the same АС frequency as the commercial mains (50 Hz in Europe, 60 Hz in North America),which means that fewer substations are necessary.
Railway systems
The foundation of а modern railway system is track which does not deteriorate under stress of traffic. Standard track in Britain comprises a flat-bottom section of rail weighing 110 lb per yard (54 kg per metre) carried on 2112 cross-sleepers per mile (1312 per km). Originally creosote-impregnated wood sleepers [cross-ties] were used, but they are now made of post-stressed concrete. This enables the rail to transmit the
pressure, perhaps as much as 20 tons/in2(3150 kg/cm2) fromthe small area of contact with the wheel, to the ground below the track formation where it is reduced through the sole plate and the sleeper to about 400 psi (28 kg/cm2). In soft ground, thick polyethylene sheets are generally placed under the ballast to prevent pumping of slurry under the weight of trains.
The rails are tilted towards one another on а 1 in 20 slоре. Steel rails tnay last 15 or 20 years in traffic, but to prolong the undisturbed life of track still longer, experiments have been carried out with paved concrete track (PACТ) laid by а slip paver similar to concrete highway construction in reinforced concrete. The foundations, if new, are similar to those for а
motorway. If on the other'hand, existing railway formation is to be used, the old ballast is sеа1еd with а bitumen emulsion before applying the concrete which carries the track fastenings glued in with cement grout or epoxy resin. The track is made resilient by use of rubber-bonded cork packings 0.4 inch (10 mm) thick. British Railways purchases rails in 60 ft (18.3 m) lengths which are shop-welded into 600 ft (183 m) lengths and then welded on site into continuous welded track with pressure-relief points at intervals of several miles. The contfnuotls welded rails make for а
steadier and less noisy ride for the passenger and reduce the tractive effort.
Signalling
The second important factor contributing to safe rail travel is the system of signalling. Originally railways relied on the time interval to ensure the safety of a succession of trains, but the defects rapidly manifested themselves, and a space interval, or the block system, was adopted, although it was not enforced legally on British passenger lines until the
Regulation of Railways Act of 1889. Semaphore signals
became universally adopted on running lines and the interlocking оf points [switches] and signals (usually accomplished mechanically by tappets) to prevent conflicting movements being signalled was also а requirement of the 1889 Асt. Lock-and-block signalling, which ensured а safe sequence of movements by electric checks, was introduced on the London, Chatham and Dover Railway in 1875.
Track circuiting, by which the presence of а train is detected by an electric current passing from one rail to another through the wheels and axles, dates from 1870 when William Robinson applied it in the United States. In England the Great Eastern Railway introduced power operation of points and signals at Spitaifields goods yard in 1899, and three years later track-circuit operation of powered signals was in operation on 30 miles (48 km) of the London and Sout Western Railway main line.
Day colour light signals, controlled automatically by the trains through track circuits, were installed on the Liverpool Overhead Railway in 1920 and four-aspect day colour lights (red, yellow, double yellow and green) were provided on Southern Railway routes from 1926 onwards. These enable drivers of high-speed trains to have а warning two block sections ahead of а possible need to stop. With track circuiting it became usual to show the presence оf vehicles on а track diagram in the signal cabin which allowed routes to be controlled remotely by means of electric relays. Today, panel
operation of considerable stretches of railway is common-рlасе; at Rugby, for instance, а signalman can control the points at а station 44 miles (71 km) away, and the signalbox at London Bridge controls movements on the busiest 150 track-miles of British Rail. By the end of the I980s, the 1500 miles (241О km) of the Southern Region of British Rail are to be controlled from 13 signalboxes. In modern panel installations the trains are not only shown on the track diagram as they move from one section to another, but the train identification number appears electronically in each section. Соmputer-assisted train description, automatic train rеporting and, at stations such as London Bridge, operation of platform indicators, is now usual.
Whether points are operated manually or by an electric point motor, they have to be prevented from moving while a train is passing over them and facing points have to be locked, аnd рroved tо Ье lосkеd (оr 'detected' ) before thе relevant signal can permit а train movement. The blades of the points have to be closed accurately (О.16 inch or 0.4 cm is the maximum tolerance) so as to avert any possibility of а wheel flange splitting the point and leading to а derailment.
Other signalling developments of recent years include completely automatic operation of simple point layouts, such as the double crossover at the Bank terminus of the British Rails's Waterloo and City underground railway. On London Тransport's underground system а plastic roll operates junctions according to the timetable by means of coded punched holes, and on the Victoria Line trains are operated automatically once the driver has pressed two buttons to indicate his readiness to start. Не also acts as the guard, controlling the opening оf thе doors, closed circuit television giving him а view along the train. The trains are controlled (for acceleration and braking) by coded impulses transmitted through the running rails to induction coils mounted on the front of the train. The absence of code impulses cuts off the current and applies the brakes; driving and speed control is covered by command spots in which а frequency of 100 Hz corresponds to one mile per hour (1.6 km/h), and l5 kHz
shuts off the current. Brake applications are so controlled that trains stop smoothly and with great accuracy at the desired place on platforms. Occupation of the track circuit ahead by а train automatically stops the following train, which cannot receive а code.
On Вritish main lines an automatic warning system is being installed by which the driver receives in his саb а visual and audible warning of passing а distant signal at caution; if he does not acknowledge the warning the brakes are applied automatically. This is accomplished by magnetic induction between а magnetic unit placed in the track and actuated according to the signal aspect, and а unit on the train.