# Rail traction: Science and limits

Where the losses hide between the wire and the wheel

Source page: https://www.materialinsights.org/energy/e_rail_traction/science/

What this tab covers What physically happens in the conversion, step by step, the science behind it, and the hard limits that physics sets on how well it can ever work.

A modern electric train turns about 80% of the electricity it takes from the wire into work at the wheels and can return roughly 10-30% of its traction energy when braking. A diesel-electric locomotive delivers about 30% of its fuel energy as useful work.

#### The conversion, step by step
**Traction transformer (AC lines)**
Electrical (25 kV AC from the contact wire) → Lower-voltage AC · about 94% (calculated from ABB data: 7.5 units lost per 100 at the wheel)^[1](https://library.e.abb.com/public/896cf517fcce4406b7a4facb6d6b7d0c/Traction_systems_high%20power_RevB_180916_web.pdf)**Traction converter**
AC or DC (750-3,000 V on DC lines) → Variable-frequency three-phase AC · about 96% with IGBTs, about 87% with older GTO thyristors (calculated)^[1](https://library.e.abb.com/public/896cf517fcce4406b7a4facb6d6b7d0c/Traction_systems_high%20power_RevB_180916_web.pdf)^[2](https://real.mtak.hu/245801/1/A9_186_%2BFischer.pdf)**Motors and gearbox**
Electrical → Mechanical (wheels) · about 89% (calculated: 12.2 units lost)^[1](https://library.e.abb.com/public/896cf517fcce4406b7a4facb6d6b7d0c/Traction_systems_high%20power_RevB_180916_web.pdf)**End to end, wire to wheel**
Electrical → Kinetic · about 80% for traction, about 70% with auxiliaries and parked use (calculated: 100 at the wheel from 124 and 143 units); older chain about 68% and 60%^[1](https://library.e.abb.com/public/896cf517fcce4406b7a4facb6d6b7d0c/Traction_systems_high%20power_RevB_180916_web.pdf)**Regenerative braking**
Kinetic → Electrical (to line or other trains) · chain efficiency about 70-85%; recovers roughly 10-30% of traction energy^[2](https://real.mtak.hu/245801/1/A9_186_%2BFischer.pdf)**Diesel-electric, fuel to work**
Chemical (diesel) → Mechanical, via engine, alternator, converter and motors · 29.1% average, Turkish State Railways diesel freight, 2005-2018^[3](https://acikerisim.bartin.edu.tr/entities/publication/56dfc2c6-0a82-4c4f-b096-e7a7af5f96ed/full)

#### The science

The train collects current through a pantograph sliding on the contact wire, or a shoe on a third rail; losses in the wire itself were not quantified in the checked sources. On AC lines a transformer steps 25,000 volts down, then fast transistor switches (IGBTs) make three-phase AC at whatever frequency the motors need. Axle motors turn this into torque through a gearbox; reversed, they act as generators and return electricity. A diesel-electric locomotive swaps the wire for a diesel engine driving an alternator, avoiding a multi-megawatt mechanical gearbox; its weak link is the engine, which must reject most fuel energy as heat. Steel wheels on rails deform little and long trains have small drag per tonne, so rail carries 8% of the world's passengers and 7% of freight with 2% of transport energy (IEA, 2019).

#### Limits set by science

| Limit | Value | Why it exists | Best today |
|---|---|---|---|
| **Electric traction chain losses** | about 20% of input in a modern chain (calculated from ABB data)^[1](https://library.e.abb.com/public/896cf517fcce4406b7a4facb6d6b7d0c/Traction_systems_high%20power_RevB_180916_web.pdf) | Copper resistance, magnetic losses in steel and switching losses can be reduced, not removed. | ABB's ICE 1 example: converter losses fell from 18.1 to 4.7 units per 100 at the wheel; total energy drawn fell from 167 to 143 units, about 15% less |
| **Regenerative braking** | regenerative chain efficiency 0.70-0.80 on DC lines, 0.75-0.85 on 25 kV AC^[2](https://real.mtak.hu/245801/1/A9_186_%2BFischer.pdf) | Braking energy is reused only if a nearby train is drawing power, the substation can feed the grid, or storage is fitted; otherwise it is burnt in resistors. | Railjet trains on 25 kV AC: about 20-30% of consumed energy regenerated; Sao Paulo Metro inverter substations cut substation energy by about 13% (9-41% by traffic) |
| **Carnot bound for the diesel engine** | about 88% (calculated: 1 - 300/2,500, assumed 2,500 K peak gas and 300 K surroundings)^[3](https://acikerisim.bartin.edu.tr/entities/publication/56dfc2c6-0a82-4c4f-b096-e7a7af5f96ed/full) | A loose bound, as the gas is this hot only briefly; wall heat loss, friction and the electrical transmission cut real output much further. | About 29% fuel to work on average in the Turkish study |
| **Electrification coverage** | 42.2% of the world's 929,981 route-km electrified (calculated from UIC data)^[4](https://img5.uic.org/IMG/pdf/synopsis_2025_web.pdf)^[5](https://www.iea.org/reports/the-future-of-rail) | Without wires, trains must carry fuel or batteries. The Americas have 0.5% electrified, Europe 56.8% and Asia Pacific 62.8% (calculated). | About three quarters of passenger rail activity runs on electric trains (IEA, 2019) |

#### Sources

1. [Traction systems for high-power applications (brochure, Rev B) (company data)](https://library.e.abb.com/public/896cf517fcce4406b7a4facb6d6b7d0c/Traction_systems_high%20power_RevB_180916_web.pdf) ABB, 2018
2. [Acceleration and regenerative braking energy of electric trains: a review for energy-management decision making (Decision Making Advances 4(1))](https://real.mtak.hu/245801/1/A9_186_%2BFischer.pdf) S. Fischer, Szechenyi Istvan University, 2026
3. [Energetic, exergetic, and environmental evaluation of railway transportation by diesel and electric locomotives (Environmental Progress and Sustainable Energy 41(3), doi 10.1002/ep.13804); Turkish State Railways freight data 2005-2018](https://acikerisim.bartin.edu.tr/entities/publication/56dfc2c6-0a82-4c4f-b096-e7a7af5f96ed/full) M. E. Akay and A. Ustaoglu (Wiley), 2022
4. [Railway Statistics Synopsis, 2025 edition](https://img5.uic.org/IMG/pdf/synopsis_2025_web.pdf) International Union of Railways (UIC), 2025
5. [The Future of Rail](https://www.iea.org/reports/the-future-of-rail) IEA, 2019

## Other sections

- [Summary](https://www.materialinsights.org/energy/e_rail_traction/index.md)
- [Overview](https://www.materialinsights.org/energy/e_rail_traction/overview/index.md)
- [Correlation](https://www.materialinsights.org/energy/e_rail_traction/correlation/index.md)
- [Value chain](https://www.materialinsights.org/energy/e_rail_traction/valuechain/index.md)
- [Unit economics](https://www.materialinsights.org/energy/e_rail_traction/economics/index.md)
- [Engineering methods](https://www.materialinsights.org/energy/e_rail_traction/methods/index.md)
- [Equipment and suppliers](https://www.materialinsights.org/energy/e_rail_traction/equipment/index.md)
- [Relationships](https://www.materialinsights.org/energy/e_rail_traction/links/index.md)