In this paper the sizing problem of supercapacitive mobile energy storage system (ESS) is considered by comparing two ESS control strategies, besides experimentally measured power diagrams of the tram Tatra T3A running in Riga city are used. Both control strategies discharge supercapacitor (SC) bank with a power that is proportional to the tram power, however, the choice of a discharge proportionality coefficient and discharge depth of SC bank for these strategies differs. Sizing is carried out by numerous simulations using the Matlab model of the system that includes the tram, ESS, substation and contact lines. The research shows that one control strategy ensures higher energy consumption decrease from the substation; however, it shortens the life expectancy of the SC bank.