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This paper proposes the transformer planning for an electrified mass rapid transit (MRT) system so that the overall cost of main transformers is minimized over the life cycle. The unit commitment is applied to derive the optimal transformer capacity to meet the annual peak demand and provide reserve for service reliability. The motion equation of train sets is used to derive the dynamic power consumption and travel distance as each time snapshot. The Simulink package is applied to solve the power demand of each train set by the simulation of VVVF inverters and induction motors. The DC/AC load flow analysis is performed to find the annual power loading of traction substations (TSS) and whole Taipei MRT network. The transformer loss, investment, and service reliability are used to define the equivalent cost of all feasible states for each dynamic programming (DP) stage. According to the computer simulation, very significant cost saving is obtained by the proposed methodology for transformer capacity planning of an MRT network.