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Originally Posted by Kelly Anderson:
Most railroaders are familiar with the tendency of AB brakes and their descendants to "self release" if the air is bottled in the car after the brakes are applied and the car spotted, and as I understand it, in a string, if one car releases, it will release some air from its reservoir into the trainline, causing the other cars in the cut to release as well.
My question is, when running a train down a long grade, an engineer will make a reduction and lap his brake valve, effectively bottling the trainline (no air being added or removed). Why aren't there instances recorded of "self-releasing" happening in those situations, causing the potential of the engineer losing control of his train?
I believe it is due to the Pressure Maintaining Feature in modern (26 and newer) brake valves.
Originally Posted by Kelly Anderson:
Most railroaders are familiar with the tendency of AB brakes and their descendants to "self release" if the air is bottled in the car after the brakes are applied and the car spotted,
First of all, when cars are spotted, the angle cock is left open to avoid a self release. I think you will find that leaving cars with both angle cocks closed is against the rules of every class one RR.
quote:My question is, when running a train down a long grade, an engineer will make a reduction and lap his brake valve, effectively bottling the trainline (no air being added or removed). Why aren't there instances recorded of "self-releasing" happening in those situations, causing the potential of the engineer losing control of his train?
Lapping the brake does not "Bottle" the trainline. It stops the equalizing reservoir from releasing any more air. Any build up of pressure OVER THE SET EQUALIZING GAUGE PRESSURE will be exhausted to atmosphere through the brake valve.
So why did I highlight the above? Because, any leakage in the trainline will cause the pressure maintaining feature (on a loco so equiped) to add air to the system in order to combat the leakage and not cause more braking to be applied to the cars.
Therein lies the rub.
Sometimes, there will be a cushioned car in the train that has a slip-joint in its trainline that has a leak in it when slack is bunched. Now, let's say that after topping a hill and descending a grade the engineer applies the dynamic brake, which bunches the slack, which causes the trainline to leak at this car. And, let's say that the leak is not so bad that the pressure maintaining feature has no problem maintaining the set air pressure.
Now, let's say that descending the grade the tonnage is so great that the dynamic brake cannot hold back the train and a brake application is needed to control the speed of the train. The brake application tends to stretch out the train as the brakes take effect. When the slack gets stretched out at the car with the leaking slip-joint, it causes the joint to slide back to where it doesn't leak. This will cause a local increase in brakepipe pressure and in turn cause the brakes to start releasing, causing some difficult train handling problems. This doesn't happen often, but, it is something an engineer needs to be aware of.
Originally Posted by Kelly Anderson:
Thanks for the answers.
No pressure maintaining and no dynamics on our RR.
But then, you don't have any ABDW and newer brake valves on your cars, do you?
I see your point about the equalizing piston relieving any pressure build up, providing that it is more sensitive than the release feature of the most sensitive car somewhere in the train.
Thanks again.
