At 01:10 PM 8/24/2000 -0500, John S. wrote:
Regarding further specifics of air brake dynamics I speak for myself, and
I suspect a great many others, when I say that while I have a basic
general knowledge of how a train's pressurized air line keeps the brakes
on individual cars released, and how angle cocks are used to retain that
pressure when cuts of cars are set out, past that I am a little bit fuzzy.
For example, how and why retainers are utilized in practice has never been
adequately explained to me.
I should probably wait for the professional Rails to answer but since I'm
at home and have a few minutes you'll have to wade through my explanation!
It may be necessary for an engineer to set and release the brakes several
times on a steep grade but when the brakes are released the train begins to
accelerate, obviously. If the engineer tries to quickly reset the brakes
he gets less braking than before because the air reservoirs have not had a
chance to recharge. It can take forever and a day to recharge the air on a
long freight train (probably seems longer when one is charging down a
mountain, I'm guessing!) so someone came up with the idea of "retaining"
some of the air in the brake cylinder after the train brake is released,
thereby keeping the brakes applied while still allowing the reservoir to
recharge. The retainer was born....
Retainers are manually operated - a crew member must walk the train and
manually set them. They have four positions AFAIK - direct release, slow
release, low-pressure hold and high-pressure hold. Usually only a
percentage of cars are set to retain - it's up to the engineer to decide
how he wants them set.
As far as your other questions - I'd better sit back and learn something
from the pros....
John Knopp
jdknopp@g...
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