This is all you need to know about locomotives

European rail is facing a major rolling stock challenge. A big part of the fleet is old — very old — and new technological developments require expensive locomotive upgrades. Fleet renewals are inevitable as the old locomotives retire. What do you need to know about locomotives, heading into the future? An explainer for those (relatively) unfamiliar with the technical and business aspects.
The world of locomotives is full of technical terms and economic considerations. RailFreight.com dove deeper into everything there is to know about them.

The technical side of things

The Chinese locomotive Shen24 boasts a power output of 28.8MW and provides a starting tractive effort of 2,280 kN. It has no less than 24 axles. Shen24 is the world’s most powerful electric locomotive, and accordingly, all of these numbers are high relative to other locomotive types. But what do they mean exactly?

Tractive effort (kN) and power output (MW)

Let’s start with tractive effort, which describes how hard a locomotive can pull a train. There is a difference between tractive effort from standstill (starting tractive effort), the maximum tractive effort and which tractive effort can be maintained indefinitely without overheating.

It is measured in kilonewtons (kN), where a higher number indicates a stronger pulling force. The kN indicator ranges from 300 kN to 1,400 for specialised heavy haul locomotives. In Europe, it is commonly between 300 and 500 kN. Shen24’s 2,280 kN starting tractive effort far exceeds the typical European traction. It can continuously provide 1,596 kN.

Power output is commonly expressed in megawatts (MW) or horsepower. If tractive effort tells you how hard a locomotive can pull, power indicates the rate at which the locomotive can apply this force to the wheels. Without getting too much into the physics behind it, high tractive effort is vital to get a heavy train moving. Once it speeds up, the challenge is no longer getting enough traction to pull it, but having enough power to keep it moving at speed. The European heavy hauler locomotive Euro9000 has a power output of 9MW.

Power sources

The energy to get the engine running has to come from somewhere. In the early days, steam locomotives used coal as their fuel. Nowadays, locomotives can run on electricity, which it draws from overhead lines or an on-board battery. Diesel locomotives are necessary on non-electrified lines (nearly 60% of railways in Europe are electrified). Click the field below to learn more about power sources.

Power sources

Steam
In the early days of rail, steam engines were used to drive the wheels. Locomotives were typically powered by burning coal, wood, or oil.

Diesel-electric and diesel-hydraulic
These locomotives use diesel fuel. Diesel-electric locomotives use a diesel engine to drive an electric generator, which in turn powers traction motors. This is the most common type of locomotive. Diesel-hydraulic locomotives use a so-called torque converter to transfer power to the wheels. They are common in heavy industrial transportation or shunting.

Electric
Powered by electricity from overhead wires, which cover around 60% of the European rail network. They are more energy efficient than diesel locomotives, converting around 90% of the energy into motion. The main downside of electric locomotives is that they cannot run on non-electrified lines. This precludes their entry into many terminals, where diesel or battery locomotives are necessary.

Dual-mode/hybrid
These locomotives can operate using two different power sources, often the overhead lines and a diesel engine. These can be used to run efficiently on electrified railways and to cover non-electrified sections, such as last-mile legs into terminals.
Hybrid locomotives can also have a battery module, which similarly allows locomotives to cover non-electrified routes.

Battery-electric
Battery-electric locomotives are powered entirely by large rechargeable batteries. These are a novelty, and their use is still being proved in practical applications.

Electrified railways (AC/DC and kV)

You have an electric locomotive. Great! Now you can operate on around 60% of Europe’s railways, at least when it concerns power. Or can you?

It turns out that there are multiple ways in which electricity flows through a circuit. There is Alternating Current (AC) and Direct Current (DC). What these are exactly is not crucial to understanding locomotives for the purposes of this explainer. The key thing to know is that DC offers the most efficient use of electricity in motors, whereas AC is the most efficient way to transfer electricity over longer distances (the overhead lines). In some countries, the overhead lines provide DC, while others provide AC.

Locomotives capable of running on AC networks convert the power to DC — a big drawback, because this needs a separate device that takes up space.

In order to operate across these systems, you need so-called multi-system locomotives. They help reduce costs and save time, because there is no longer a need to change locomotives at the border. Besides the AC/DC differences, electricity networks can also operate with varying voltages. Some examples from Europe:

• 25 kV AC (common in France, UK, parts of Eastern Europe)
• 15 kV AC (Germany, Austria, Switzerland, Scandinavia)
• 3 kV DC (Poland, Belgium, Italy)
• 1.5 kV DC (Netherlands, parts of France)

Four or six axles?

If you have ever looked into locomotives before, you might have noticed that the number of axles on locomotives can differ. Most locomotives have either four or six axles. The more axles a locomotive has, the more it spreads its weight across the wheels. This is important, because rail infrastructure typically has a maximum axle load, meaning how much weight can rest on a single axle. If this weight exceeds the indicated maximum, it could cause damage to the rail infrastructure.

A higher number of axles means that the locomotive can be heavier before exceeding this limit. At the same time, the extra set of powered wheels provides additional starting tractive effort, making them better suited for heavy tonnage. The electrical and thermal loads are also better distributed, preventing the motors from overheating on steep inclines. Six axles therefore facilitate heavy hauls better than four axles.

Four-axle locomotives perform better at higher speeds, flat terrain and non-heavy haul (intermodal) operations. They also require less maintenance.

Wheel arrangements

Axles are attached to bogies, which are mounted on the locomotive. Four-axles locomotives have two bogies with two axles. Six-axles locomotive can have two bogies with three axles each, or three bogies with two axles each. This is where the wheel arrangements come in.

The symbolic abbreviation “Bo” often denotes a two-axle bogie. A “Bo-Bo” locomotive is a four-axle locomotive where the axles are mounted across two bogies. “Co” represents a bogie with three axles. Many six-axle locomotives have a “Co-Co” wheel arrangement.

There are, of course, more wheel arrangements. See a non-exhaustive list by collapsing the box below.

Locomotive wheel arrangements

Bo-Bo: Four axles mounted in two bogies.

Co-Co: Six axles mounted in two bogies.

A1A-A1A: An uncommon six-axle configuration where only the outer axles of each three-axle bogie are powered, leaving the center axle unpowered. It helps weight distribution while reducing bogie oscillation.

Bo-Bo-Bo: Instead of two large three-axle bogies, the locomotive rides on three smaller 2-axle bogies. The Soviet VL85 freight locomotive uses a Bo-Bo-Bo+Bo-Bo-Bo wheel arrangement.

Co-Bo: A highly specific asymmetrical layout with one three-axle powered bogie and one two-axle powered bogie. This is used on the Japanese diesel-hydraulic locomotive DE10.

The economic or business side of things

Locomotives are pricy. Very pricy, in fact. Locomotive deals or funding frequently reach into the dozens of millions of euros. In other words, acquiring locomotives is a major capital investment, and therefore not always the optimal choice.

Owning your own locomotives can offer real advantages: you have full control over them, you can customise them as desired and it provides long-term availability. The downside is that you are responsible for ensuring maintenance, and it requires a lot of money upfront to acquire them in the first place. Why not spend that money on launching new services, expanding in other areas?

That is why leasing can be an attractive option. Rather than owning the locomotives directly, you pay a periodic fee to a locomotive lessor. This can offer financial flexibility, as you can more easily upscale or downsize your fleet (and therefore costs). It also makes it possible to expand more rapidly, as the big capital investments can go towards other areas.

Besides a simple lease, sometimes operators secure the option to buy a locomotive at a later stage. Full-service leasing is another variety, where the lessor provides maintenance and technical support as well.

Manufacturers and locomotive families

Locomotives are manufactured by, you guessed it, manufacturers. European manufacturers include companies like Alstom, Stadler and Siemens Mobility. CRRC is China’s state-owned manufacturer, whereas in the USA Wabtec and Progress Rail are well-known producers of locomotives.

The three European manufacturers above have spawned locomotive “families” that are common throughout all of Europe. These families comprise similar locomotives made by the same company.

Europe’s biggest locomotive family is the Siemens Vectron family. Siemens Mobility has sold over 2,900 locomotives of the Vectron family to over 110 customers. In total, the fleet has covered around 1.35 billion kilometres in service. Locomotives based on the Vectron platform have been approved for operation in 20 European countries.

The Bombardier/Alstom TRAXX family is only slightly outpaces by the Vectrons. Some 2,500 units had been sold globally as of 2023. Then there is Stadler’s ‘EURO‘ family, which now counts 400 locomotives in 12 European countries, as well as in Uruguay and Türkiye.