Electrified Railways as Flexible Resources for Grid Support: A Structured Narrative Review

The increasing penetration of renewable energy sources has led to a rising demand for fast frequency response, flexible demand-side resources, reserve capacity, and low-inertia support in the power system. Due to their high traction power, rapid controllability, adjustable operating trajectories, multi-train aggregation potential, and regenerative braking capability, Electrified railways are increasingly regarded as potential grid-support resources. This structured narrative review explores how electrified railways support the operation of the power system through various mechanisms and technical approaches. The evidence base is established through structured multi-database searches, targeted supplementary searches, as well as forward and backward citation tracking. The reviewed studies are categorized according to four grid-service timescales: inertia-like and fast-frequency support targeting the millisecond-to-second service timescale, primary frequency regulation over seconds to minutes, secondary regulation and reserve provision over minutes to hours, and energy management over hours to a day. The reviewed evidence indicates that trajectory-based traction-power regulation is currently the most developed mechanism in direct high-speed-rail T2G research. However, the broader flexibility of electrified railways also involves controllable auxiliary loads, regenerative braking, traction substations, onboard and wayside storage, renewable generation, railway microgrids, and coordinated fleet-level resources. These resources operate through different mechanisms and should not be represented by a single trajectory-centered model. The available grid support capabilities must be evaluated under specific technical and operational constraints of the resources, including punctuality, speed limits, safety intervals, passenger comfort, traction network conditions, converter and energy storage limitations, as well as ATC/ATO compatibility. Despite significant theoretical potential, practical application is still limited by insufficient field validation, cross-system coordination challenges, unclear market mechanisms, communication requirements, and incomplete integration with storage and renewable generation. Future research should give priority to practical demonstrations in the real world, coordinated railway–power-system operation, control strategies that are compatible with the market, and source–grid–train–storage integration.