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[quote user="Ulrich"] Getting back to my original point, we in the trucking industry use a SPRING brake mechanism which is activated when air pressure in the system DECREASES. It works in a manner opposite to how the service brakes work. Air pressure is used to compress a heavy duty spring on each wheel. When the air pressure in the system drops the air in the brake chamber containing the spring can no longer hold the spring, and the spring brake is applied. I see no reason why a brake like that could not be adapted for railway purposes. You simply need to  bleed air out of the system through a valve on the locomotive in order for all "parking" brakes on the train  to apply instantaneously. .  It works like a charm on trucks,  and I've  have had the dubious pleasure of first hand experience to attest to that. <div style="clear:both;">[/quote]</div> That is a natural idea for a fail-safe brake.  If you lose the air, the brakes still set.  The original railroad air brakes were straight air, and the obvious problem was that if the train breaks in two, you lose all braking on both halves because the train line is open at the break.  So Westinghouse invented the triple valve, and a whole new pneumatic strategy that applies the air brakes by reducing the train line pressure, as opposed to the straight air system where you pressurize the train line to apply the brakes.  The spring loaded cylinder system would also apply with a reduction of train line pressure.  Whatever all the pros and cons of the spring system versus the triple valve system, the industry adopted the triple valve system at a time when the spring system was an obvious alternative.  I don’t think that the spring system will replace the triple valve system at this point.  The advantage of the spring system over the triple valve system is that the springs will not disengage over time like the triple valve system will disengage due to leakage.  Assuming the current air brake system is here to stay, a parking brake could still be developed as an addition that would leave the standard air brake system intact.  I am sure there a bunch of patents of workable systems of parking brakes.  While a parking brake system should be an add-on to the standard air brake system, it could still utilize the pneumatic cylinders of the standard air brake system for the application of brakes when functioning as the parking brakes.  But when you disengage the add-on parking brake system, it leaves the standard air brakes to function as they presently do.
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