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New electrically propelled passenger trains use AC propulsion with induction traction motors and variable voltage, variable frequency drives. Benefits include higher performance, reduced starting energy use, regenerated braking energy, lower maintenance, and a broadened range of operating conditions. However, some transit operators have found that new trains have not fully delivered the economic benefits of regeneration. An effective energy storage system (ESS) can capture the full energy savings benefit of regenerative braking and will reduce the peak power usage which incurs high peak power demand charges. Candidate ESS technologies include flywheels, supercapacitors, and batteries. This paper provides a set of typical requirements for a transit wayside ESS, and evaluates the strengths and weaknesses of the candidate technologies against the requirements. Primary ESS benefits are energy savings and peak power reduction. The authors calculated potential savings for a generic ESS at two typical transit systems: a dense US heavy rail system with 10-car trains, and a US light rail line that runs trains of two to four light rail vehicles (LRVs). In the heavy rail case study, a properly sized ESS will provide energy savings of up to 4.4 kWH per train start/run/stop cycle. This is a savings of 3.6 MWH per day per ESS, or 21% of the total energy. In the light rail case study, ESS will reduce the peak power demand by up to 119 kWH/H, or 23% of the total peak demand.