In current non-electrified rail systems there is a significant loss of energy during vehicle braking. The aim of this research has been to investigate the potential benefits of introducing onboard regenerative braking systems to rail vehicles. An overview of energy saving measures proposed within the rail industry is presented along with a review of different energy storage devices and systems developed for both rail and automotive applications. Advanced flywheels have been identified as a candidate energy storage device for rail applications, combining high specific power and energy. In order to assess the potential benefits of energy storage systems in rail vehicles, a computational model of a conventional regional diesel train has been developed. This has been used to define a base level of vehicle performance, and to investigate the effects of energy efficient control strategies focussing on the application of coasting prior to braking. The impact of these measures on both the requirements of an energy storage system and the potential benefits of a hybrid train have been assessed. A detailed study of a range of existing and novel mechanical flywheel transmissions has been performed. The interaction between the flywheel, transmission and vehicle is investigated using a novel application-independent analysis method which has been developed to characterise and compare the performance of different systems. The results of this analysis produce general ‘design tools’ for each flywheel transmission configuration, allowing appropriate system configurations and parameters to be identified for a particular application. More detailed computational models of the best performing systems have been developed and integrated with the conventional regional diesel train model. The performance of proposed flywheel hybrid regional trains has been assessed using realistic component losses and journey profiles, and the fuel saving relative to a conventional train quantified for a range of energy storage capacities and power-train control strategies.