How Trains Work

Air brakes

Most North American freight and passenger trains use a continuous air-brake system. The locomotive maintains pressure in a brake pipe that runs through the train. Changes in brake-pipe pressure cause control valves on the cars to apply or release the brakes.

This design is fail-safe in an important way: a major loss of brake-pipe pressure can trigger an emergency brake application rather than leaving the train with no braking.

Couplers & draft gear

Automatic knuckle couplers connect cars and locomotives. The coupler and its draft gear must handle both draft forces when the train is being pulled and buff forces when cars compress together.

Managing those forces is a major reason train handling matters. Sudden changes in throttle or braking can create large forces through a long train.

Distributed power

Not every locomotive has to be on the front. Distributed power units (DPUs) can be placed in the middle or rear of a train and controlled from the lead locomotive.

DPUs can improve train handling by spreading tractive effort and braking effort through the consist. They can also reduce extreme in-train forces on long, heavy trains, especially on grades and undulating territory.

Signals

Railroad signals communicate movement authority and operating conditions to crews. Signal aspects can indicate whether a train may proceed, must reduce speed, or must be prepared to stop. Exact meanings vary by railroad, territory and rule system.

Dispatching

Train dispatchers coordinate train movements over a territory. They line routes, manage conflicts, issue authorities or instructions, and work with crews and other railroad employees to keep traffic moving safely.

On busy main lines, dispatching can resemble air-traffic control for trains: many movements share a limited number of tracks, sidings, junctions and terminals.

Railroad yards

Some yards use gravity and a raised hump to sort cars; others use flat switching with locomotives. Yard design depends on railroad needs, traffic and available space.

Grades & curves

A grade is the slope of the track. Even a grade that looks small on paper can have a huge effect on a heavy train because the resistance acts across the entire train.

Curves add resistance too. Tight curvature increases the effort required to move a train and can limit speed. Railroads therefore pay close attention to ruling grades, curvature and route profile when deciding how much locomotive power a train needs.

Train makeup

Where cars are placed in a train matters. Railroads consider car weight, length, commodity, braking characteristics, destination, special handling requirements and operating rules.

Train makeup affects handling because a long consist behaves like a chain under tension and compression. Heavy cars, light cars, empty cars and long cars can react differently to curves, grades and braking.

How a train starts moving

1. The crew confirms authority and operating conditions.
2. Brakes are released and brake-pipe pressure stabilizes.
3. Power is applied gradually.
4. Slack may stretch through the train as couplers transition from compressed to stretched.
5. The crew monitors speed, traction, signals, grade and train response.

On a very long freight train, the rear of the train does not necessarily begin moving at exactly the same instant as the head end. Slack action can travel through the consist as couplers stretch.

Dynamic braking

On many diesel-electric locomotives, traction motors can be used as generators during braking. The resulting electrical energy is typically dissipated as heat through resistor grids on conventional locomotives. Dynamic braking reduces wear on friction brakes and is especially valuable on long descents.

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