On old ballast cars, the laborers walked along the cars and operated the doors. Later the operator sat on a seat which hooked onto the car's grab irons. He had a remote control, which opened the doors. These had solar panels to charge the batteries, which powered the doors.
This Herzog train is GPS based. A couple of years ago, I was the engineer of the work train. The Herzog guy set up in the rear unit. His rig had a laptop, and a GPS antenna. There is a train line cable which signals the opening of the cars. The line is mapped out before. This way the stone is dropped in the required locations. It will avoid places like open bridges. It even keeps track of how much stone has been dropped from each car. This way the system will move to the next car, when the previous one is empty. The only drawback was that we had to back up and hit one area again. This was because the line is right next to a hillside. The hill blocked out the signal from space, so we had to hit it again in manual mode.
The GPS system can allow speeds to about 35mph. On my run they were also dumping in the gauge, so we were limited to 10 mph. The real drawback it that it starts with the car on the head pin, and works back. This makes handling the train trickier.
