A method for determining a torsional moment of a wheel set shaft of a rail vehicle during the operation of the rail vehicle is used for a wheel set shaft having two wheels secured to ends of the shaft for rolling on two rails. A model is used to calculate a torsional moment which acts on the wheel set shaft, and the model is based on a torsional vibration of the wheel set shaft at a specified slip action point. The torsional moment acting on the wheel set shaft is ascertained based on the energy of the torsional vibration of the wheel set shaft at the slip action point and based on a damping energy which acts on the torsional vibration of the wheel set shaft.