From the window of the machine by oscarps

Machinist is the person in charge of controlling the speed, braking, as well as the closing and opening of doors, and the systems of aboard a locomotive, train unit, automotive, and therefore, any railway vehicle.
In the time of the vapor formed inseparable pair with a stoker; Were in charge of a locomotive on an ongoing basis. With the disappearance of steam traction, the figure of the stoker disappears, being replaced by that of the assistant engineer. For many years the machinists exert their functions with the assistant, being this a previous step to become machinist. However, since 2001, with the law that comes into force on the sole agent, the train drivers start to work alone, and the helper disappears completely.

@oscarps