A method for operating a train system comprising the steps of: generating power by a power source; transmitting the power generated by the power source through a drive arrangement including: an input shaft connected to the power source; an output shaft; and a hybrid gearbox comprising layshaft gearbox, connected to the input shaft and to the output shaft, and comprising a star carrier configured to adjust the transmission speed ratio of the layshaft gearbox between the input shaft and the output shaft, and a speed ratio motor-generator unit, configured to control the rotation of the star carrier; driving an equipment connected to the output shaft; and operating the speed ratio motor-generator unit to start up the power source, and then to decouple the load from the power source, to adjust the rotation of the star carrier to set the transmission speed ratio of the layshaft gearbox so as to adjust the speed ( ω o) and the torque ( C o) at the output shaft when the power source is operated at constant speed ( ω i), characterized in that the power source is a single-shaft gas turbine, wherein the operating step comprises the sub-steps of: operating the speed ratio motor generator unit as a motor to bring the gas turbine to its crank speed; switching-off the speed ratio motor generator unit to decouple the single-shaft gas turbine to accelerate and to reach its nominal speed, so that the star carrier is free to spin; and operating the speed ratio motor-generator unit as generator and slow-down the star carrier to increase the output shaft speed from stand-still condition. generating power by a power source; transmitting the power generated by the power source through a drive arrangement including: an input shaft connected to the power source; an output shaft; and a hybrid gearbox comprising layshaft gearbox, connected to the input shaft and to the output shaft, and comprising a star carrier configured to adjust the transmission speed ratio of the layshaft gearbox between the input shaft and the output shaft, and a speed ratio motor-generator unit, configured to control the rotation of the star carrier; an input shaft connected to the power source; an output shaft; and a hybrid gearbox comprising layshaft gearbox, connected to the input shaft and to the output shaft, and comprising a star carrier configured to adjust the transmission speed ratio of the layshaft gearbox between the input shaft and the output shaft, and a speed ratio motor-generator unit, configured to control the rotation of the star carrier; layshaft gearbox, connected to the input shaft and to the output shaft, and comprising a star carrier configured to adjust the transmission speed ratio of the layshaft gearbox between the input shaft and the output shaft, and a speed ratio motor-generator unit, configured to control the rotation of the star carrier; driving an equipment connected to the output shaft; and operating the speed ratio motor-generator unit to start up the power source, and then to decouple the load from the power source, to adjust the rotation of the star carrier to set the transmission speed ratio of the layshaft gearbox so as to adjust the speed ( ω o) and the torque ( C o) at the output shaft when the power source is operated at constant speed ( ω i), characterized in that the power source is a single-shaft gas turbine, wherein the operating step comprises the sub-steps of: operating the speed ratio motor generator unit as a motor to bring the gas turbine to its crank speed; switching-off the speed ratio motor generator unit to decouple the single-shaft gas turbine to accelerate and to reach its nominal speed, so that the star carrier is free to spin; and operating the speed ratio motor-generator unit as generator and slow-down the star carrier to increase the output shaft speed from stand-still condition. operating the speed ratio motor generator unit as a motor to bring the gas turbine to its crank speed; switching-off the speed ratio motor generator unit to decouple the single-shaft gas turbine to accelerate and to reach its nominal speed, so that the star carrier is free to spin; and operating the speed ratio motor-generator unit as generator and slow-down the star carrier to increase the output shaft speed from stand-still condition.