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In this paper, a railway power conditioner (RPC) based on modular multilevel converter (MMC) with split supercapacitor energy storage system (SCESS) is studied. In this case, the MMC-SCESS based RPC could not only provide normal negative sequence currents (NSC) compensation but also could reduce the impact of power fluctuations caused by the locomotive braking or startup on the electric grid. Firstly, this paper analyzes the power flow patterns and deduces the reference circulating current under different operation modes of MMC-SCESS based RPC. Then, the control objectives of MMC-SCESS based RPC are divided into two categories including the balance control and the current tracking control. The balance control methods are developed for the submodules (SMs) capacitor voltages and the state of charge (SoC) of the supercapacitor, which are associated with the operation modes. To ensure the current tracking performance, a model predictive direct current control (MPDCC) method is presented for MMC and bidirectional energy converter (BEC). Finally, the effectiveness of the proposed control methods is verified by the experimental results of a downscaled prototype. Index Terms-Railway power conditioner (RPC), modular multilevel converter (MMC), split supercapacitor energy storage system (SCESS), power flow patterns, balance control, model predictive direct current control (MPDCC).