This is the back of a tractor. The hitch has a locking mechanism that prevents the top of the hitch from opening without pulling up the little T at the back of hitch. And that loop is for the chains to attach to. Obviously there is another one on the other side.
The back of our trailers are similar, with a pin that goes through the hitch, to keep the hitch from opening, as one final redundancy to keep the dolly attached.
One final thing about our hookups: during feeder school they talked about how secure the trailer pin is when it is hooked up to the fifth-wheel. They told us that nothing can detach the two when they are properly snapped together, even during a roll-over. That seemed hard to believe, but sure enough, we've had guys drop the back trailer off the road and have that trailer roll over. The front trailer stays upright and that dolly never disconnects. What happens is the eye of the dolly is designed to turn, so even when the back trailer is on its side, the fifth wheel of the dolly is still connected to the trailer. It is just on its side.
Obviously, this equipment is designed much better than that little rinky-dink trailer you guys are to forced to use. Typical.