Abstract Rail potential is affected by a variety of influence factors in the urban rail transit. This paper proposes a state space average (SSA) model of the traction power system to explore the effects caused by the traction network resistance, inductance, capacitance and rail-to-ground resistance. The SSA model solutions prove that the traction network inductance and capacitance have significant influences on rail potentials. Moreover, based on the SSA model, the rail potential distribution of a metro line during the train timetable period is obtained for rail protections, which is conductive to improve the reliability of the traction power system.