Material Insights · Understanding the Material World

Railway electrification and wireless power: How it works

Why 25 kV on a wire beats carrying fuel on board

What this tab covers How the energy actually moves, step by step: the science that makes it work and the engineering that makes it practical.

Electric trains take power continuously from a wire overhead or a third rail beside the track, so they never carry fuel. Wireless systems instead pass energy across an air gap by magnetic fields or beams of light, and battery swapping moves energy by physically carrying charged batteries to the vehicle.

Step by step

  1. Grid feeds traction substationsSubstations along the line take power from the grid and supply it to the track at a standard voltage. The EU interoperability rules allow four systems: 25 kV AC at 50 Hz (the usual choice for new main lines), 15 kV AC at 16.7 Hz (Germany, Austria, Switzerland, Scandinavia), and 3 kV or 1.5 kV DC; metros and some suburban networks use lower DC voltages, such as 750 V third rail in southern England.
  2. Contact wire or third rail carries the currentA copper contact wire hung from a catenary (a supporting cable) or a steel rail beside the track carries current along the route; the running rails return it. Low-voltage DC remains common on metros and older suburban networks, while high-voltage AC suits long, heavily used routes because it needs fewer substations.
  3. Train collects and uses the powerA pantograph on the roof presses against the wire (or a shoe slides on the third rail), and power electronics feed the traction motors. When braking, the motors work as generators and push power back into the line for other trains.
  4. Wireless charging across a gapFor road vehicles, a coil buried in the road or a parking pad carries a rapidly changing current; its magnetic field induces a current in a receiver coil under the vehicle. Oak Ridge National Laboratory sent 100 kW across a gap of about 13 cm (five inches) into a Hyundai Kona at 96% efficiency in 2024.
  5. Beaming by laserA laser sends a narrow beam to a receiver of photovoltaic cells tuned to its light. In 2025 a DARPA programme delivered more than 800 W over 8.6 km for 30 seconds.
  6. Carrying the battery insteadIn battery swapping, a robot removes a depleted pack and fits a charged one in about five minutes; packs are recharged at the station from the grid.

The science

Losses in a wire rise with the square of the current (P = I² R). A 25 kV line carries the same power as a 750 V line with about one thirty-third of the current, so for the same conductor it loses far less in heat and needs substations much less often (calculated from the ratio of voltages); a UK engineering seminar put AC energy use about 15% below DC for similar duties. Wireless charging relies on Faraday's law: a changing magnetic field induces a voltage in a nearby coil. Tuning both coils to resonate at the same frequency lets energy pass efficiently across a gap of several centimetres, but efficiency drops quickly as the gap widens or the coils misalign. Laser beaming loses energy twice, turning electricity into light and light back into electricity, plus scattering in the air.

The engineering

Overhead lines need masts at regular intervals, tensioning weights to keep the wire straight in heat and cold, neutral sections where different supply phases meet, and enough clearance under bridges and in tunnels, which is often the costliest part of electrifying an old line. AC traction supplies are single phase, because collecting three phases from overhead wires is impractical. Electric roads either hang catenary over lorry lanes or bury segmented coils that switch on only when a fitted vehicle passes. Swap stations combine robotics, a battery store and chargers.

Key numbers

Main-line overhead voltage (new lines) 25 kV AC, 50 Hz2 Other EU systems 15 kV AC at 16.7 Hz; 3 kV and 1.5 kV DC2 Passenger rail activity on electric trains about three-quarters in 2016 (up from 60% in 2000)1 Share of lines electrified, UIC survey members about 42% of the 930,000 km worked by UIC survey members (calculated); nearly 57% in the EU (2022)1223 Static wireless charging record (ORNL, 2024) 100 kW at 96% efficiency, about 13 cm gap5 Laser beaming record (DARPA, 2025) over 800 W at 8.6 km8

Sources

  1. The Future of Rail (data to 2016) IEA, 2019
  2. TSI Energy: technical specification for interoperability of the energy subsystem, Regulation (EU) No 1301/2014 (presentation) European Union Agency for Railways (ERA), 2020
  3. System energy optimisation strategies for DC railway traction power networks (PhD thesis) University of Birmingham, 2017
  4. Novel technology reaches 100-kW wireless power transfer for passenger vehicle Oak Ridge National Laboratory, 2024
  5. DARPA program sets distance record for power beaming DARPA, 2025
  6. China is betting on battery swaps to tackle EV chargers shortage Rest of World, 2024
  7. Railway Statistics Synopsis, 2025 edition International Union of Railways (UIC), 2025
  8. Has third rail had its day? Rail Engineer, 2013
  9. Electrified railway lines increased by 31% since 1990 Eurostat, 2024