A drive train comprises an internal combustion engine, a clutchless gearbox, which can be shifted by means of an electronic control unit and by means of actuating devices, without a mechanical synchronizing device, and a hydrostatic gearbox with a hydraulic pump and a hydraulic motor, which are connected in a closed circuit. The rotational speed of the hydraulic motor being controllable in dependence on a control signal to the adjusting device of the hydraulic pump and/or to the adjusting device of the hydraulic motor. The control unit calculates the control signal by means of setpoint input via an accelerator pedal or as a function of the rotational drive speed of the internal combustion engine, with shifting of the transmission stages being possible by means of the actuating devices. The control unit controls the adjusting device of the hydraulic pump as well as the actuating devices for the gearbox for the electronic synchronization of the latter by means of a characteristic curve which represents the speed ratio of the hydraulic motor to the hydraulic pump in dependence on an actuating signal to the adjusting device of the hydraulic pump for a loadfree state of the gearbox. For this purpose, in the method of controlling the drive train, the control unit calculates a maximum achievable synchronous speed of the hydraulic motor by means of a characteristic curve which represents the speed ratio of the hydraulic motor to the hydraulic pump in dependence on an actuating signal to the adjusting device of the hydraulic pump and/or to the adjusting device of the hydraulic motor for a loadfree state of the gearbox.