Yo Steve.
Yes all modern Multiple Units built since the 1960s have auto-parking brakes. (As I'm sure you know) usually on each driving motor car there is an extra cylinder at the back of the brake cylinder which has a piston and a spring. This is fed from the brake reservoir, such that when working pressure is available the piston holds the spring back. But if the reservoir empties and there is no air available for the brake, the spring pushes forward onto the brake mechanism and moves the disc pad onto the wheel or axle, depending on the mechanics of the brake. This is not terribly strong - about the equivalent of a 'step one' brake application, but is enough to hold a train on a gradient, or bring it to a stand eventually.
The APB is a really great addition, because if a train is stabled unattended and the juice goes off for any reason (which it does regularly overnight round here), the compressor won't work and the train will slowly lose its air. As it does so the parking brake brake will apply and the train stays where it is. Better stilll, as the first driver of the day finds that all the trains in the yard have shut down, he'll be running around getting them to 'pump up'. As the air pressure on-board starts to rise and the main brakes start to apply again, the parking brakes will take themselves off. It all happens silently and out of sight, but prevents the embarrassment and danger of having trains running away on their own.
God bless the brake fairies.
Hope this makes sense.
Cheers, BW.