pyotr filipivich wrote:
on Tue, 15 Jun 2010 14:12:22 -0400
typed in rec.crafts.metalworking the following:

The important safety features of the Westinghouse brake system are that
1) each car has it's own reservoir, and THAT air is used to apply that
car's brakes, independently of any other car. and 2) that a DROP in air
pressure in the train line applies the brakes. Hence, in the event of a
separation of the train line (a break-in-two of the train for example),
the pressure loss applies the brakes.

My understanding has been that the air pressure keeps the brakes
"open" (off), and that loss of air pressure applies the brakes. Sort
of a fail safe mode.

Air pressure above a certain point releases the
brakes.

The engineer pumps up the train brake line to
release the brakes and fill the small reservoirs
on each car. Then when he "sets some air" by
blowing down the pressure on the brake line, the
three-way valve on the car channels air pressure
from the reservoir to the wheel brake cylinders.
The more he sets in one application, the higher
the pressure applied to the wheel brakes.

Breaking the airline or putting the brake system
into "emergency" applies the full reservoir pressure
to the wheel brakes.

Now a clever listener might ask "Then how can the
engineer start a train that's stopped on a grade?"