A method of operating a train system for driving a mechanical driven equipment is disclosed. The train system comprises a hybrid gearbox connected between a power source and a load to be driven. The hybrid gearbox includes a lay shaft gear, which transmission ratio between the power source and the load and a motor-generator unit can be adjusted, to adjust the transmission speed ratio arranged, to balance the power generated by the power source and to improve the efficiency of the train system in a number of operating conditions.