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.

司机orak language(only bus driver,car driver)， 驾驶员 official language(include pilot driver deck officer)

Generally they are the same. . For taxia, truck, bus, cars, we often use 司机 For rocketa, aeroplanes we often use 驾驶员 but sometimes they are not interchangeable For example 坐在驾驶员( emphasize the action–driving)旁边的是一个出租车司机( emphasize his profession) The man sitting beside the driver is a taxi driver

驾驶员是书面语，口语中不说驾驶员。😊

In my experience, both of them are drivers. 司机 is more common and also is far more often used to refer to the person and the Job. in daily life.