Chris
You're in luck, that is a Truax Patent mine Orecar, Google patent #466,717 1892 or #545,433 1895, the drawings are online. The spring loaded treadle plate at the rear disengages the dumpbody from the travelling position and allows the swivel, the reach arms on the side control the opening of the endgate.
I haven't yet come across a car that gripped the rails when dumping though. If you look at your first picture you will see a timber across the dumprails to prevent an end-off. Some mines actually bent their dump rails into a halfcircle to catch the orecar wheels.