In a method for heating an operating agent for a rail vehicle, particularly for heating a reducing agent for the after-treatment of exhaust gas, a coolant liquid is pumped through a cooling circuit of the internal combustion engine by a pump when an operating agent heating system is in an operating mode. In the operating mode, the coolant liquid is pumpable through a main heating circuit by of the pump in order to heat the operating agent in a reservoir. When the operating agent heating system is in a preheating mode, the main heating circuit is divided into two sub-circuits of a preheating circuit, the flow being able to pass through both of the sub-circuits.