How trains work

1

From steam to electric

Every train turns stored energy into turning wheels. What changed over two centuries is where the energy is kept and how well it's used. A steam engine burns its fuel on board and wastes most of it as heat. A diesel-electric also burns fuel on board, but far more efficiently, and uses electricity to get the power to the wheels. An electric train carries no fuel at all: it takes its power from the line.

• Steam1804Trevithick's locomotive hauls a train at Penydarren, in South Wales.
• Steam1825The Stockton and Darlington Railway opens: the first public railway to use steam locomotives.
• Steam1863The Metropolitan Railway opens under London: the world's first underground railway, worked by steam.
• Electric1890The City and South London Railway opens: the first deep-level tube, worked by electric trains from the start.
• Electric1920s–30sThe Southern Railway electrifies its suburban lines with a third rail.
• Diesel1955British Railways' Modernisation Plan sets out to replace steam with diesel and electric trains.
• Diesel1968British Railways runs its last steam passenger trains.
• Diesel1976The High Speed Train enters service, and diesel trains reach 125 mph (201 km/h) in everyday running.
• Electric2007High Speed 1 opens in full, and trains from St Pancras run at up to 186 mph (299 km/h).

How much of the fuel's energy reaches the rails

Steam: about 8% of the coal's energy reaches the cylinders, and three-quarters of that the drawbar, for engines from 1930 on. Diesel: today's figure; in 1960 it was about 23%. Both from a 2024 study of the world's railways (Tostes and others, Applied Energy). *The electric train's figure is an estimate, counted from the wire: the same study takes 90% from the motors to the wheels, but no source read gives one figure for the whole train. Power stations and the grid lose some energy before it reaches the wire, but an electric train also gets some back: it can return power to the line when it brakes.

Next: Steam ↓

2

Steam: fire, water and a piston

A steam locomotive is a boiler on wheels. A coal fire heats water until it boils, and the steam, trapped and under pressure, pushes a piston. Rods turn the piston's back-and-forth push into the turning of the wheels. The engine below is a Metropolitan Railway A Class tank engine, cut open. The Metropolitan opened in 1863 with borrowed engines; the A Class, from 1864, were its own.

Most of the heat goes up the chimney or warms the air, which is why a steam engine turns less than a tenth of its coal's energy into pulling power. It also needs hours to raise steam, regular stops for water, and a fireman shovelling tonnes of coal.

Next: Diesel-electric ↓

3

Diesel-electric: a power station on wheels

The figure below is a Class 68, built in Valencia from 2013, first by Vossloh España and later by Stadler, which took the works over, and used on both passenger and freight trains. A diesel engine is far more efficient than steam, but it has a weakness: it can't pull from a standstill, and it gives its best power only over a narrow range of speeds. A car gets round this with a clutch and gearbox. A gearbox for a Class 68's 2,800 kilowatts (about 3,750 horsepower) would be enormous, so most diesel locomotives don't use one. Instead, the engine turns a generator, and the electricity drives motors on the axles. An electric motor pulls hardest from a standstill and needs no gear changes.

Most of Britain's diesel multiple units (the Sprinters and most Turbostars, for example) use a hydraulic transmission instead: a torque converter, in which oil driven by the engine turns the output, takes the place of the generator and motors. Some, like the Class 172 Turbostar, have a mechanical gearbox, as a car does. Bi-mode trains carry both a diesel engine and a pantograph, and use whichever the line allows.

Next: Electric ↓

4

Electric: power from the line

The figure below shows two cars of a Class 387 Electrostar, a four-car electric unit built to run on both overhead wires and third rail. An electric train carries no engine and no fuel. It collects electricity from a wire overhead or a rail beside the track, and its motors turn the wheels. With no engine to carry, it's lighter for its power, so it can accelerate harder. And when it brakes, its motors can work as generators and send power back into the line for other trains to use.

Next: Third rail or overhead ↓

5

Third rail or overhead line?

Britain uses two main systems. South of London and on Merseyside, trains collect 750 volts DC from a third rail beside the track. Most other electrified main lines, and High Speed 1, use 25,000 volts AC from an overhead wire, and High Speed 2 will too. The Underground has a fourth rail as well, between the running rails, so that the return current doesn't stray into tunnel linings.

Why the voltage matters

Power is voltage times current. A third rail's 750 volts is about a thirtieth of 25,000, so for the same power it must carry about 33 times the current. The heat lost in a conductor grows with the square of the current. So a third rail has to be a heavy conductor, and it needs far more substations to feed it than an overhead line does.

Next: Going round bends ↓

6

Going round bends

A train has no steering wheel, and its wheels are fixed to a solid axle, so both turn at the same rate. Yet on a bend the outer rail is longer than the inner one. The trick is in the shape of the wheels. Their treads aren't flat but slightly coned: larger in diameter next to the flange, smaller towards the outside.

On a bend the wheelset drifts outwards. The outer wheel then rolls on a larger part of its cone, so it travels further with each turn, and the inner wheel rolls on a smaller part. The axle steers itself round the curve.

Each wheel also has a flange: a lip on its inner edge, which runs just inside the rail. If the wheelset slides too far sideways, on a very tight curve or through a set of points, the flange presses against the side of the rail and holds the train on the track. Normally the coned treads keep the flanges clear, so they're a safety net rather than the steering.

Cant: leaning into the curve

To keep passengers comfortable and the load even on both rails, the outer rail on a curve is raised. This is cant (a road engineer would call it banking). At just the right speed, the push outwards and gravity add up to a force straight down through the floor, and you feel nothing. Go faster and you feel pushed outwards: the shortfall is called cant deficiency. Tilting trains lean their bodies further in, so they can take curves faster.

Next: Heat and cold ↓

7

Rails in heat and cold

Steel grows as it warms. A 60-foot rail gets about a fifth of a millimetre longer for every degree, and in summer sun a rail can be far hotter than the air. Older track was laid in short lengths bolted together with fishplates, leaving a small gap at each joint so the rails had room to grow. Wheels crossing those gaps make the clickety-clack.

Most main lines now use continuous welded rail: lengths welded into one rail that can run for miles. It has no room to grow, so it's held down hard by concrete sleepers, clips and ballast, and the heat becomes force instead. To keep that force manageable, the rail is stretched as it's laid so that it's under no stress at about 27 °C. Colder than that, it's being pulled; hotter, it's being squeezed. Too hot, and it can buckle sideways. On the hottest days, speed limits are imposed until the rails cool, and some rails are painted white to keep them cooler.

Next: Noise ↓

8

Why wheels are sometimes noisy

A train running on smooth, welded track is quiet. Most of the noise you hear comes from something being slightly wrong at the point where the wheel meets the rail.

• Clickety-clack: wheels crossing the gaps at the joints of older, jointed track. The sound comes in pairs because a bogie has two axles. Welded track has no joints, so no clacks.
• Squeal on tight curves: on a sharp bend the coned treads can't make up the whole difference between the inner and outer rails, and a bogie's two axles, fixed parallel, can't both line up with the curve. The wheels are dragged sideways across the railhead in a fast stick-slip, and a wheel, usually the leading inner one, rings at one of its own notes, high and piercing. The leading outer wheel's flange rubs the rail and adds a hiss. You hear it most on tight curves. Treating the rail head or lubricating the flange quietens it.
• A rhythmic thump: a wheel that has slid under braking, often on leaves, wears a flat spot. It bangs once a turn until the wheel is reprofiled on a wheel lathe.
• A roar: corrugation, ripples worn into the top of the rail. Grinding trains smooth them away.
• Brake squeal as the brake pads or blocks grip.

Next: Leaves on the line ↓

9

Leaves on the line

It sounds like an excuse, but it's real physics. Wet leaves that fall on the line are crushed by passing wheels into a thin black film that bakes onto the rail. It's slippery as ice: a wheel can lose almost all its grip. Braking trains slide past platforms and signals, and starting trains spin their wheels. The film can even insulate the wheels from the rails, so the signalling briefly loses track of where a train is.

The railway fights back each autumn. Railhead treatment trains blast the film off with high-pressure water jets and lay a gel of sand on the rails. Trains carry sanders that drop sand in front of their wheels, and wheel slide protection eases the brakes off a sliding wheel and on again, like a car's ABS. Lineside trees are cut back, and timetables allow extra time.

Next: Stopping ↓

10

How long trains take to stop

A car's rubber tyres grip tarmac hard. A train's steel wheels on steel rails grip far less: that's what makes trains so efficient to keep rolling, and so slow to stop. A train also can't brake harder than its wheels can grip without sliding, and sliding wheels wear flat spots and stop no sooner. Add hundreds of tonnes, and a train at 125 mph (201 km/h) needs well over a kilometre, most of a mile, even with its emergency brake. So the signals for 125 mph (201 km/h) lines are spaced for about 2 km.

Grip

How hard a wheel can push or brake before it slips, as a share of the weight on it, and what a train's brakes are designed to need:

That is why leaves matter so much: trains are designed to brake without needing more grip than about 0.1, and a leaf film can leave far less, as little as an ice skate on ice (see Leaves on the line).