The increasing electrification of urban public transports requires improving the design of the electrical infrastructures to take into account all the technical and financial challenges involved in the creation of a new line. This paper presents a new optimization tool dedicated to the sizing of tramway electrical infrastructures: power substations, overhead transmission lines, feeders and equipotential bonding. The purpose is to determine the number, positions and technical characteristics of all these components to achieve the best trade-offs between investment costs, energy costs and the quality of the traffic power supply. The sizing problem is formulated as a multiobjective optimization problem and solved using the NSGA-II genetic algorithm. The proposed method is applied to a simple test case and gives good results.