The fact that they were empty should make the brakes work better. Train cars are supposed to have brakes that turn on when they lose air so that they won't roll downhill if they are accidentally detached. Maybe someone who knows more about trains than me can comment on how this happened.
Thanks
wideminded,
Angle cocks were turned on two cars then the pin was lifted between the two, uncoupling the cars. By turning the angle cocks, the air is "bottled" in the cut and the brakes will not set when the train line is separated. Had the angle cocks not been closed, the cut of cars would have likely not rolled more than a few hundred feet because the train line would have been "broken" and the brakes would set up in an "emergency application". Hope that helps..