Follow a train’s energy
Trace energy through a simplified electric railway. Some energy can return during braking, but neither the train nor its power network is lossless.
Adapt the incoming supply
The traction supply receives electricity and adapts it for the railway. Direct-current and alternating-current railways use different feeding and conversion equipment. The example begins at this interface with the wider electricity network; it does not assume that every train system uses the same voltage or arrangement of substations. Its local arrangement sets the electrical interface.
Distinguish the railway’s supply arrangement from the electricity arriving at its grid connection.
Conversion and feeding equipment must match the train system.
Read the complete journey
- 1
Adapt the incoming supply
The traction supply receives electricity and adapts it for the railway. Direct-current and alternating-current railways use different feeding and conversion equipment. The example begins at this interface with the wider electricity network; it does not assume that every train system uses the same voltage or arrangement of substations. Its local arrangement sets the electrical interface.
Notice: Distinguish the railway’s supply arrangement from the electricity arriving at its grid connection.
Explore Traction power
Why: Conversion and feeding equipment must match the train system. - 2
Turn electricity into motion
A current collector transfers energy to the moving train, and traction equipment converts electrical energy into mechanical work. Acceleration increases the train’s kinetic energy. Energy is also used by onboard services and lost through physical and electrical processes, so the entire incoming supply does not become recoverable motion. Energy accounting includes those services and losses.
Notice: Separate energy added to motion from energy used by onboard services and conversion losses.
Explore The train
Why: Only the motion-related portion is a candidate for recovery during the later stop. - 3
Travel through real constraints
The train’s movement is shaped by the route and its permitted speed. Hills, curves and the next stop affect the sequence of acceleration, coasting and braking. Signalling can also require a change in motion. The energy used by a journey therefore depends on its operating pattern, not just its distance.
Notice: Compare acceleration, coasting and braking phases rather than treating distance as the only input.
Explore Track and support
Why: Operating conditions influence the energy required for the journey. - 4
Convert some motion back
During regenerative braking, suitable motors can generate electrical energy while the train slows. This recovers part of the energy associated with motion. Braking still needs to function when regeneration cannot do all the work; the train’s overall braking arrangements are designed around safe stopping, not a promise of maximum recovery.
Notice: Trace an energy arrow out of the braking train while keeping the stopping requirement in view.
Explore Braking and energy recovery
Why: Energy recovery is useful only within an arrangement that still provides the required braking. - 5
Find a receptive destination
Returned energy needs a destination. Another compatible train drawing power nearby may use it, or suitable equipment may route it to storage or another electrical supply. If the network cannot accept it, it cannot simply accumulate without limit in the conductor. Recovery opportunities depend on the electrical arrangement and service pattern.
Notice: Look for a receptive destination at the same time as braking energy becomes available.
Explore Traction power
Why: An electrical connection alone does not guarantee that returned energy can be used. - 6
Connect energy to the service pattern
Departures and stops determine when trains draw and may return energy. That creates a link between the timetable and the traction supply. It does not mean energy recovery should dictate every departure: capacity, passenger needs, safe separation and reliable operation remain part of the same connected design problem. No single objective sets the whole schedule.
Notice: Compare when trains accelerate with when other trains brake.
Explore Timetables and headway
Why: The service pattern affects demand and recovery opportunities across the traction network.
Why does regenerative braking not guarantee that all stopping energy returns to the public grid?
Reveal the explanation
Only part of the energy is recoverable, and it needs a receptive destination. The train, nearby loads, storage and substation arrangement determine where useful energy can go; other braking and losses remain part of the energy balance.
Sources & further reading
Original explanations informed by these sources. Diagrams are conceptual; local systems differ.
- Third rail Network Rail · Not stated · Source & rights record
- Electrification Network Rail · Not stated · Source & rights record
- Recycled energy on Tube trains (2) London Assembly / Mayor of London · 2015-11-23 · Source & rights record
- The timetable Network Rail · Not stated · Source & rights record
- Transit Capacity and Quality of Service Manual, Third Edition, Chapter 8: Rail Transit Capacity Transportation Research Board / National Academies · 2013 · Source & rights record
Another way through the system.
Take a train across the city
Follow the passenger, then reveal the systems underneath.
When a longer stop travels down the line
Follow a small change through a connected timetable.
Before the first passenger boards
Follow a train from preparation to its first useful departure.
After heavy rain, before normal service
Follow water through the railway’s connected dependencies.