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This paper presents an analysis of the energy storage requirements for hybrid railway vehicles. Autonomous hybrid railway vehicles combine two power sources. The conventional diesel primemover is supplemented with power derived from an on-board energy storage device. The device is discharged during high power demand, and may be recharged through regenerative braking. The energy savings that are achievable through the adoption of such technology are also dependent on the duty cycle. It is shown that for a given vehicle there is a inter-station distance beyond which the benefits become marginal.The performance of the energy storage device fundamentally affects the whole system performance and careful consideration is required in device selection. The constrained nature of the railway system also presents opportunities for optimization of architectures and control strategies. These methods are also discussed in this paper. Finally, a review of practical implementation considerations is presented. Hybrid railway vehiclesThe principle of a hybrid propulsion system is to use more than one power source for vehicle propulsion. Choices of hybrid architecture and system configuration depend on the vehicle duty cycle, and also depend on issues such as whole life cycle costs and maintainability. For systems with two power sources, the prime mover is usually an internal combustion engine, and this is supported by another power source, such as a battery system during periods of high power demand. The path of power * Corresponding author, +44 (0)121 414 4289 s.hillmansen@bham.ac.uk from prime mover to the wheels of the vehicle also have many technically feasible options. Railway vehicles have a number of systems in use currently, including diesel electric transmission which is common in many locomotives, and increasingly a feasible option for multiple units. In principle, only minor propulsion system modifications are required to convert an existing electric transmission system into one which can accommodate electrical energy storage between the traction drives and the prime mover.Energy savings from a system containing energy storage can be realized through the downsizing and optimization of the prime mover, and through the capture and release of braking energy. Railway operations also have further potential options for energy savings by optimizing the driving style to maximize the use of regenerated energy, and by careful management of the energy storage device.The selection of an energy storage medium is a complex process, and devices may have fundamentally different characteristics, meaning the resulting final systems differ considerably. Electric storage medium include supercapacitors, batteries, and magnetic field storage. Each of these technologies have different operating constraints and will potentially each have niche applications. Mechanical storage includes compressed storage systems, and flywheel systems. This paper presents a broad analysis of hybrid railway vehicle configurations as shown in figure 1, wi...