What’s the difference between 驾驶员 and 司机?

both have a subtle difference. 司机 is generally used for medium or small vehicles. for example, train, truck, bus, car. 驾驶员 is usually used for large vehicles or aircrafts. for example, train, tank, airplane. and all called 司机 can be called 驾驶员. but some of 驾驶员 can be seldom called 司机, for example airplane.