Control software, which is intended for a multiplicity of rail vehicles, contains basic functions that are required for the basic operation of the rail vehicles. In addition, optional functions are implemented in the control software which are required for carrying out customer-specific requests. The basic functions and the optional functions are tested, validated and approved in their specifications, combinations and functional processes prior to installation in the rail vehicles. The basic functions and the optional functions are then implemented in the rail vehicles. In a selected rail vehicle, at least one optional function is activated or deactivated using a switching parameter which is individually associated with the optional function. The required switching parameter is created outside of the rail vehicle and subsequently transferred to the selected rail vehicle. The switching parameter is transferred encrypted to the selected rail vehicle, selectively specific to an individual vehicle.