layout and gave greater freedom to designers.

In 1825 only 18 steam locomotives were doing useful work. One of the

first commercial railways, the Liverpool & Manchester, was being built, and

the directors had still not decided between locomotives and саblе haulage,

with railside steam engines pulling the cables. They organized а

competition which was won by Stephenson in 1829, with his famous engine,

the Rocket, now in London's Science Museum.

Locomotive boilers had already evolved from а simple

flue to а return-flue type, and then to а tubular design, in which а nest

of fire tubes, giving more heating surface, ran from the firebox tube-plate

to а similar tube-plate at the smokebox end. In the smokebox the exhaust

steam from the cylinders created а blast on its way to the chimney which

kept the fire up when the engine was moving. When the locomotive was

stationary а blower was used, creating а blast from а ring оf perforated

pipe into which steam was directed. А further development, the multitubular

boiler, was patented by Henry Booth, treasurer of the Liverpool &

Manchester, in 1827. It was incorporated by Stephenson in the Rocket, after

much trial and error in making the ferrules of the copper tubes to give

water-tight joints in the tube

plates.

After 1830 the steam locomotive assumed its familiar form, with the

cylinders level or slightly inclined at the smokebox end and the fireman's

stand at the firebox end.

As soon as the cylinders and axles were nо longer fixed in or under the

boiler itself, it became necessary to provide а frame to hold the various

components together. The bar frame was used on the early British

locomotives and exported to America; the Americans kept со the bar-frame

design, which evolved from wrought iron to cast steel construction, with

the cylinders mounted outside the frame. The bar frame was superseded in

Britain by the plate frame, with cylinders inside the frame, spring

suspension (coil or laminated) for the frames and axleboxes (lubricated

bearings) to hold the

axles.

As British railways nearly all produced their own designs, а great many

characteristic types developed. Some designs with cylinders inside the

frame transmitted the motion to crank-shaped axles rather than to eccentric

pivots on the outside of the drive wheels; there were also compound

locomotives, with the steam passing from а first cylinder or cylinders to

another set of larger ones.

When steel came into use for building boilers after 1860, higher

operating pressures became possible. By the end of the nineteenth century

175 psi (12 bar) was common, with 200 psi (13.8 bar) for compound

locomotives. This rose to 250 psi (17.2 bar) later in the steam era. (By

contrast, Stephenson's Rocket only developed 50 psi, 3.4 bar.) In the l890s

express engines had cylinders up to 20 inches (51 cm) in diameter with а 26

inch (66 cm) stroke. Later diameters increased to 32 inches (81 cm) in

places like the USA, where there was more room, and locomotives and rolling

stock in general were built larger.

Supplies of fuel and water were carried on а separate tender, pulled

behind the locomotive. The first tank engine carrying its own supplies,

appeared tn the I830s; on the continent of Europe they were. confusingly

called tender engines. Separate tenders continued to be common because they

made possible much longer runs. While the fireman stoked the firebox, the

boiler had to be replenished with water by some means under his control;

early engines had pumps running off the axle, but there was always the

difficulty that the engine had to be running. The injector was invented in

1859. Steam from the boiler (or latterly, exhaus steam) went through а

cone-shaped jet and lifted the water into the boiler against the greater

pressure there through energy imparted in condensation. А clack (non-return

valve)

retained the steam in the boiler.

Early locomotives burned wood in America, but coal in Britain. As

British railway Acts began to include penalties for emission of dirty black

smoke, many engines were built after 1829 to burn coke. Under Matthetty

Kirtley on the Midland Railway the brick arch in the firebox and deflector

plates were developed to direct the hot gases from the coal to pass over

the flames, so that а relatively clean blast came out of

the chimney and the cheaper fuel could be burnt. After 1860 this simple

expedient was universа11у adopted. Fireboxes were protected by being

surrounded with а water jacket; stays about four inches (10 cm) apart

supported the inner firebox from the outer.

Steam was distributed to the pistons by means of valves. The valve gear

provided for the valves to uncover the ports at different parts of the

stroke, so varying the cut-off to provide for expansion of steam already

admitted to the cylinders and to give lead or cushioning by letting the

steam in about 0.8 inch (3 mm) from the end of the stroke to begin the

reciprocating motion again. The valve gear also provided for reversing by

admitting steam to the opposite side of the piston.

Long-lap or long-travel valves gave wide-open ports for the exhaust

even when early cut-оff was used, whereas with short travel at early cut-

off, exhaust and emission openings became smaller so that at speeds of over

60 mph (96 kph) one-third of the ehergy of the steam was expanded just

getting in and out of the cylinder. This elementary fact was not

universal1y

accepted until about 1925 because it was felt that too much extra wear

would occur with long-travel valve layouts.

Valvе operation on most early British locomotives was by Stephenson

link motion, dependent on two eccentrics on the driving ах1е connected by

rods to the top and bottom of an expansion link. А block in the link,

connected to the reversing lever under the control of the driver, imparted

the reciprocating motion tо the valve spindle. With the block at the top of

the link, the engine would be in full forward gear and steam would be

admitted to the cylinder for perhaps 75% of the stoke. As the engine was

notched up by moving the lever back over its serrations (like the handbrake

lever of а саr), the cut-off was shortened; in mid-gear there was no steam

admission to the cylinder and with the block at the bottom of the link the

engine was in full reverse.

Walschaert's valvegear, invented in 1844 and in general use after 1890,

allowed more precise adjustment and easier operation for the driver. An

eccentric rod worked from а return crank by the driving axle operated the

expansion link; the block imparted the movement to the valve spindle, but

the movement was modified by а combination lever from а crosshead on the

piston rod.

Steam was collected as dry as possible along the top of the boiler in а

perforated pipe, or from а point above the boiler in а dome, and passed to

а regulator which controlled its distribution. The most spectacular

development of steam locomotives for heavy haulage and high speed runs was

the introduction of superheating. А return tube, taking the steam back

towards the firebox and forward again to а header at the front end of the

boiler through an enlarged flue-tube, was invented by Wilhelm Schmidt of

Cassel, and modified by other designers. The first use of such equipment in

Britain was in 1906 and immediately the savings in fuel and especially

water were remarkable. Steam at 175 psi, for example, was generated

'saturated' at 371'F (188'С); by adding 200'F (93'C) of superheat, the

steam expanded much more readily in the cylinders, so that twentieth-

century locomotives were able to work at high speeds at cut-offs as short

as 15%. Steel tyres, glass fibre boiler lagging, long-lap piston valves,

direct steam passage and superheating all contributed to the last

phase of steam locomotive performance.

Steam from the boiler was also for other purposes.

Steam sanding was introduced for traction in 1887 on th

Midland Railway, to improve adhesion better than gravity

sanding, which often blew away. Continuous brakes were

operated by а vacuum created on the engine or by соmpressed air supplied by

а steam pump. Steam heat was piped to the carriages, arid steam dynamos

[generators] provided electric light.

Steam locomotives are classified according to the number of wheels.

Except for small engines used in marshalling уаrds, all modern steam

locomotives had leading wheels on a pivoted bogie or truck to help guide

them around сurves. The trailing wheels helped carry the weight of the

firebox. For many years the 'American standard' locomotive was a 4-4-0,

having four leading wheels, four driving wheels and no trailing wheels. The

famous Civil War locomotive, the General, was а 4-4-0, as was the New York

Central Engine No 999, which set а speed record о1 112.5 mph (181 kph) in

1893. Later, а common freight locomotive configuration was the Mikado type,

а 2-8-2.

А Continental classification counts axles instead оf wheels, and

another modification gives drive wheels а letter of the alphabet, so the 2-

8-2 would be 1-4-1 in France and IDI in Germany.

The largest steam locomotives were articulated, with two sets of drive

wheels and cylinders using а common boiler. The sets оf drive wheels were

separated by а pivot; otherwise such а large engine could not have

negotiated curves. The largest ever built was the Union Pacific Big Вoу, а

4-8-8-4, used to haul freight in the mountains of the western United

States. Even though it was articulated it could not run on sharp curves. It

weighed nearly 600 tons, compared to less than five tons for Stephenson's

Rocket.

Steam engines could take а lot of hard use, but they are now obsolete,

replaced by electric and especially diesel-electric locomotives. Because of

heat losses and incomplete combustion of fuel, their thermal efficiеncу was

rarely more than 6%.

Diesel locomotives

Diesel locomotives are most commonly diesel-electric. А diesel engine

drives а dynamo [generator] which provides power for electric motors which

turn the

drive wheels, usually through а pinion gear driving а ring gear on the

axle. The first diesel-electric propelled rail car was built in 1913, and

after World War 2 they replaced steam engines completely, except where

electrification of railways is economical.

Diesel locomotives have several advantages over steam engines. They are

instantly ready for service, and can be shut down completely for short

рeriods, whereas it takes some time to heat the water in the steam engine,

especially in cold weather, and the fire must be kept up while the steam

engine is on standby. The diesel can go further without servicing, as it

consumes nо water; its thermal efficiency is four times as high, which

means further savings of fuel. Acceleration and

high-speed running are smoother with а diesel, which means less wear on

rails and roadbed. The economic reasons for turning to diesels were

overwhelming after the war, especially in North America, where the railways

were in direct competition with road haulage over very long distances.

Electric traction

The first electric-powered rail car was built in 1834, but early

electric cars were battery powered, and the batteries were heavy and

required frequent recharging. Тоdау е1есtriс trains are not self-contained,

which means that they get their power from overhead wires or from а third

rail. The power for the traction motors is collected from the third rail

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

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