To add to what Sean just said...let's take a typical PTC car as an example. The dogs are more visible on Arrow trains, but Arrow does strange things like using only two sets of dogs per train, etc.
PTC puts a cast assembly at the back of each car, this casting has three channels in it and a single axle running through it. At the center of this casting, there is a long steel finger with a curved notch on the end. This is the chain clutch. It can move forward over a roller chain, but not backward, and this allows the chain to pull the train to the top of the lift; it also enables a clean release at the top. The two outboard channels contain fingers that hang down and catch the sawtooth on the outer edges of the chain guide. These are the anti-rollback dogs and they prevent the train from rolling backwards down the hill. On most coasters, if the lift motor were to stop, the weight of the train would pull the chain backwards down the hill if it weren't for the anti-rollback dogs. Also, if the chain breaks, the chain slides backwards out from under the train, as the chain clutch will allow it to go freely just as it does when the train goes over the crest of the lift.
Some coasters add another dog, a "chain safety dog" on the lift to prevent the chain from moving backward and piling up at the bottom of the lift. On PKI's Vortex this is the noisy thing you can hear about halfway up the lift.
I may have some pictures around here someplace; if not, you might check and see if there are any photos of chain clutches on www.coasterquest.com .
--Dave Althoff, Jr.
