GB rail electrification


Diesel vs electric

As with other transport modes, if the objective is to reduce carbon emissions, old diesel trains can be replaced with new ones; alternatively, existing diesel engines can be replaced with modern ones. Modern engines are much more efficient, so this can significantly reduce emissions (and save on fuel costs). However, if the objective is to remove diesel altogether, then this does not solve the problem and is merely a stopgap. Similarly, increasing numbers of diesel bi-mode trains are appearing on British lines. These can run on both electrified and unelectrified lines, using the external power source where available and the on-board diesel supply when not. These are clearly an improvement on running diesels under overhead wires (OLE), but are also not a long-term solution and will have to be replaced by non-polluting sources. In any case, you can accomplish the same thing by getting passengers to change trains when the power source changes, so the main advantage of bi-modes is the convenience for passengers of not having to do that. The only real solution is to replace all diesel engines with electric.

Electric motors are simpler than diesel and therefore require less maintenance; they are lighter, generally faster and quieter, with better acceleration, besides not polluting at point of use. They also have the advantage of being able to make use of regenerative braking: by using the motor in reverse, it acts as a generator, converting the kinetic energy of the vehicle into power which can be returned to an external power supply or stored, either electrically (in a capacitor), chemically (in a battery), or mechanically (in a flywheel). This also reduces the use of frictional brakes, which in turn reduces the harmful particulates these produce.

Most transport modes have to carry the power supply with them, traditionally in the form of some fossil fuel, combusted to produce power. Rail is unusual in being to a large extent powered by an external power supply. In principle, roads could be electrified too. Trams and trolley-buses use external power, generally from overhead wires. But, in general, the cost of installing this on all roads for all vehicles would be prohibitive. All transport modes could, like satellites, use solar power for at least some of their power needs, and the Byron Bay train south of Brisbane recharges its batteries entirely from solar cells. Perhaps in the future, this may become more mainstream.

Power from outside: electrification of track

This has increasingly been used in most European countries in recent decades, though currently only Switzerland's track is fully electrified. In most places, this takes the form of overhead cabling, though Britain is unusual in having a large amount (roughly one-third) of third-rail (DC) electrification, mainly in the SE but also around Liverpool. This is lower-powered, and so means trains have lower top speeds; it also complicates train design, as any train on a route with both types of supply has to be able to draw power from both sources.


Power on the train

As stated above, it makes sense for trains with an electric motor to make optimum use of regenerative braking:

In the absence of external recharging facilities, train routes with longer stretches of unelectrified track need to carry an auxiliary power source to recharge the battery. This can be:

These technologies are not mutually exclusive. Batteries can be added to electric trains to extend an electrified route on to an unelectrified branch line. They can then be replaced with a purely electric train if the branch line is later electrified. Equipping an electric train with batteries also enables partial track electrification, and means that depots and sidings do not have to be electrified, reducing the danger to workers.

Energy: diesel vs battery vs capacitor

Diagram from Researchgate

The diagram (from Researchgate) provides a clear rough comparison of the different energy sources. Energy density is the amount of energy stored - how big the bottle is - whereas power density is the amount of energy that can flow in and out at once - how big the bottle opening is. Traditional combustion engines score comparatively well on both counts. Super/ultracapacitors can deliver (or accept) a lot of power at one go, but the power soon dissipates through the large 'opening'. Li-ion has the best combination of current battery types, having a reasonable size of both bottle and opening. They are thus usable for smaller/lighter vehicles which don't need large power. The higher the energy density, the further the range for a given size or the smaller/lighter it can be for the same range. Large HGVs or freight trains on the other hand need more power and/or a larger/heavier battery.

Solid-state batteries (i.e. those that use solid materials instead of liquid) potentially increase the energy density manyfold.

Battery vs green hydrogen

Although, as shown above, hydrogen has a better energy density than current batteries (and liquid hydrogen even better), it does not occur naturally, and has to be manufactured. Fully decarbonised 'green' hydrogen is produced from electricity by water electrolysis, and then converted back into power by reversing the process. This means that a large amount of the initial power input is lost in the process, as shown in the diagram below (from Transport & Environment). It's likely that electrolysis will improve in efficiency over the next few years, but how viable it will be in the longer term remains to be seen.

Diagram from

Future developments

So the aim of current research is to merge these technologies into new types of storage which combine the advantages of each type. This won't happen overnight, but it seems a safe bet that those in 10 years time will be much changed from existing ones. They may also be substantially cheaper per unit of power.

Research at present is concentrating on: