conductor's compartment
A small room or compartment on a train where the conductor works, typically located at the end of a car.
The door to the conductor's compartment says 'Authorized Personnel Only'.
運転室 is the driver's cab (front of the train), whereas 車掌室 is the conductor's compartment (often at the rear or middle).
Compound of 車掌 (conductor) + 室 (room, compartment).