Many countries in Europe and Asia have begun to adopt electric trains. They’re fast, smooth and efficient. Some countries like the UK, USA and Canada have chosen to stick with diesel locomotives. Since electrifying all transportation systems seems like a brilliant idea, I was wondering what makes them so good and how electric trains actually work?
The Basics: Cornering
In order to understand how electric trains work, it is important to know the basics of trains. As you will (hopefully) already know, trains run on parallel tracks, removing the need for them to steer anywhere. Many new tracks built as straight as possible in order to accommodate faster trains. In the UK, things are slightly different. Since all our rail is old, it was built back when trains were relatively slow. Locomotives didn’t have the need for straight tracks and the rail companies opted for the more convenient route of locating near housing.
As a consequence of windy tracks, trains are unable to travel at high speeds along with them due to Newton’s first law of inertia. In short, this states that an object will keep moving in a state of uniform motion unless an external force is applied. In this case, that’s the force of the rails acting on the wheels to push the train round the corner.
If the train is moving faster, it requires a greater force to change its direction. This is shown through Newton’s second law F = MA. The greater the mass of the train, the greater the force needed to accelerate it. Acceleration is a vector, meaning it not only has a magnitude, but also a direction. So, when the train goes around the corner, it may stay at the same speed, but change direction, meaning it is constantly accelerating when going around the corner.
Diesel locomotives are known for their slow starts and tug at higher speeds. This is due to the torque and power properties of the engine rather than an electric motor.
However, although pure diesel trains are still fairly common, many trains on the lines use a hybrid of a diesel engine as the “power source” and an electrical generator/electric motors in place of a gearbox. This allows them to get the power and reliability of the diesel engines as well as the speed and acceleration of the electric motor.
Unlike hybrid cars, no batteries are used, just a clever mechanism which uses the internal combustion engine to drive a generator. This generator produces AC electricity. After that, it passes through a transformer, changing it to the required voltage to drive the traction motors. Despite the fact that the electricity leaving the transformer is the right voltage, it is AC, meaning it cannot be used to drive the motors.
To (quite literally) rectify this, a rectifier circuit is used to convert the AC to DC electricity. A similar process is used in Tesla Superchargers to convert the AC electricity from the grid to DC electricity to store into batteries. In contrast to the efficient transformer, this process usually involves some kind of energy loss.
Finally, the DC electricity passes through an inverter, turning it into three-phase alternating current. That seems counter-intuitive, especially considering that the two circuits seem to cancel each other out. However, the single-phase AC which was generated by the generator is not capable of supplying the traction motors with enough power. Instead, the three-phase AC system can deliver more power, making it more suitable. This is shown in the graphs below.
Types of AC Power
Once this three-phase AC power has been generated, it is used to drive the traction motors which drive the wheels. Single-phase AC power looks like a standard sine curve, whilst three-phase AC power is three sine curves intertwined.
Carriages come in two forms; passenger and freight. Freight trains often have powerful locomotive with kilometres of carriages, making them very efficient. This is because all of the cargo space is filled. Freight cars may have special attachments for different loads, depending on what they are carrying.
By contrast, passenger trains usually only carry as many passenger cars as necessary to accommodate all of the passengers on board. In the UK, each car is often full, causing many passengers to be standing. Unloaded, passenger carriages are usually heavier than freight carriages due to the extra structure and amenities they carry.
To explore how electric trains work, we need to understand how carriages are held together by carefully designed systems which increase comfort and safety. The mechanism which holds them together is called a railroad coupling.
Historically, buffers and a three-link chain were used to connect carriages. As you can imagine, this would result in the cars frequently banging together. Currently, a C shaped coupling is used, resulting in limited free movement between the cars whilst dampening sudden jolts.
As you can see in the picture above, two C shaped claws interlock each other, creating a connection which safely attaches the carriages. Due to its rigidity, it also allows the electricity cables to travel from carriage to carriage.
This is electricity is used to power lights, braking and sometimes entertainment in the carriage. Freight trains may not have electricity links between the carriages due to the lack of need.
Most passenger trains are standard, containing seats in rows, some with tables. On longer journeys, food and drinks can be provided by the crew. However, there may also be a dining car which people can go to to get food.
Some trains are for the purposes of leisure, rather than commuting. For example, the rocky mountaineer travels through the Canadian Rockies in the BC area. The roof is open glass, allowing passengers to get an open view of the landscape as they cruise through. This type of train tends to be more luxury due to the extended journey times of up to a few days.
How do Electric Trains Receive Power?
One major part of explaining how electric trains work is how they get their power in the first place. Many other forms of locomotive are fairly simple, either coal is shovelled onto the train or diesel is pumped into the fuel tank.
Fortunately, electric trains use a relatively simple power delivery method too. They can either take power from a third rail or an overhead cable. The electrified third rail has a multitude of advantages and disadvantages:
- A third rail is cheap to construct
- Easy to build
- Simple to maintain
- Efficient for low voltage systems
- Good for short runs
- Exclusively low voltage DC. Higher voltages would be dangerous.
- Poor power transmission over long distances, requiring more frequent substation infrastructure
- Not necessarily compatible with high speeds.
- A low-lying third rail is a safety concern
Consequently, they are perfect for short-distance travel. For example, the London Underground or New York Subway. Another use case of the third rail is airport transport.
Due to the twists and turns of a city environment, the trains are unlikely to hit high-speed. Moreover, power is plentiful, making the third rail the most practical and feasible option.
Overhead Catenary Systems
The other method of getting power to the train is using overhead cables (catenary system). These are most common on long-distance routes like freight routes or the channel tunnel from England to France.
- Will run with either DC or AC power
- Are able to use higher voltages
- Can transmit power over greater distances, especially AC power
- They are better suited for high-speed operations
- Better suited for heavy freight drags
- Better suited for longer routes
- The overhead wire is away from the ground level, thus making it safer.
- Costly to build
- More complex
- More labour-intensive to maintain
- Does not transmit low voltage DC as efficiently
- Vertical clearances can be an issue
Seemingly, because overhead cables can provide AC power, they are far better for freight routes which travel long distances, so pretty much all freight routes. Although they are more costly, they’re much more practical for this type of system.
Okay, so now you should be aware of many of the vital roles of trains; movement, carriages, power, propulsion. You should also understand the basics of how electric trains work. However, there’s one more important consideration; the environment.
In a time of rapid change to the climate, many industries are making an effort to become cleaner. For example, electric cars have come into existence.
Similarly to electric cars over gasoline-powered cars, electric trains are better for the environment than their fuel-powered counterparts.
Although the cost of electrification of all lines is a large one, the environmental benefits of doing it would be larger. An additional benefit would be that electric trains don’t need large batteries, releasing strain on global lithium reserves slightly.
Do you think we should fully electrify the rail network?