All aboard: how railways can lower transport emissions

The transport sector is responsible for around a quarter of greenhouse gas emissions in Europe and is the main cause of high air pollution in cities. Increasing reliance on rail transport for short passenger journeys, freight and international travel can help the industry take a significant step towards reducing these levels. While we’re already beginning to electrify, this must continue to roll out on a greater scale.

On the right track

Rail transport is already a more efficient way of moving a large amount of people than using individual road vehicles. The high passenger occupancy of trains means that carbon dioxide (CO2) emissions per passenger are significantly reduced. In addition, less cars on the road also lessens localised congestion and pollution levels.

However, it’s not just the daily commute that can benefit from these carbon reductions. There’s a growing movement to ditch air travel for rail, with tagskyrt, which is Swedish for train-bragging, becoming a popular buzzword.

Significant CO2 reductions can also be demonstrated by transferring freight transport from roads to railways. Each tonne of freight transported by train rather than heavy goods vehicle decreases emissions by 76 percent, and removes up to 76 lorries from roads.

The ticket to carbon free

While already widely viewed as an eco-friendly option, technology developments in hydrogen electric, battery electric and overhead electric trains have the potential to cut rail travel emissions even further.

The safety and reliability of the electric railway system is dependent on the power quality of the train power supply system. Power issues such as overvoltages and harmonics can damage equipment and therefore disrupt the rail transport system. Therefore, the management of the electrical power must be carefully considered in mass-scale electrification of railways.

Resistors for railways

Overvoltages in the power supply commonly stem from lightning strikes or switching operations, but their effects can be avoidable. Soft and hard crowbar resistors can be used in traction power supply circuits to deal with the consequences of transient or longer lasting over-voltage conditions.

A soft crowbar resistor is pulsed to dissipate transient over-voltages, but if these become worse or prolonged, the main breakers are opened and the system is short-circuited through the hard crowbar to absorb the stored energy.

To prevent surges and unwanted harmonics entering the rail power supply, capacitor/inductor filter circuits are used to decouple the traction power supply from the drives. Here resistors are used to limit inrush currents to the capacitors during charging and to safely discharge them when required.

High speed trains require a large amount of energy to brake, and often disk brakes alone are unsuitable because of high wear rates with resulting maintenance and replacement costs. Many electric trains now use the electric traction motor as a generator to slow the vehicle.

Where possible, the generated electrical power is fed back into the supply line to be used by other trains elsewhere on the network in a process known as regenerative braking. However, when there are no other trains available to use the regenerated power, the excess is safely dissipated by brake resistors mounted on the train itself or at fixed track-side locations.

As the demand for freight and passenger transport grows, electrifying the railways and extending their use will be key in delivering a high-performance service with minimal environmental impact.