Thanks “Dude” for answering the question.
Yet, even those button whistles use enough air to cause a brake release. That is the kind they were using on the cab I described. It doesn’t take much of an increase in pressure (after the valve closes) to cause the brakes to release.
This discussion about air at the end of the train brings a question to mind, how does all this work when you have pusher/helper locomotives on the rear of the train. Are they hooked into the train airline and are they also adding pressure? Can they also modulate the brakes?
Rear end and/or mid-train manned helper engines have their brake systems configured to act as just another car in the train. Helpers without a device known as ‘Helper Link’ must stop to disconnect the helper and close the trainline on train itself. With Helper Link the Helper can be disconnected on the fly, as the Helper Link ‘connects the trainline’ without a physical connection, but a radio controlled trainline connection. For manned mid-train helpers the train must be stopped to insert and remove the helper. Manned helpers are on trains for additional power, not brake control.
As Balt says, the brakestand on any locomotive not in control of the train is cut out, so that locomotive is just another boxcar, brake-wise.
The independent still works, though.
But if the helper, midtrain and/or rear, is radio controlled from the head end, then it does help by shortening the time the specific application, pressure reduction or the build-up for brake release, travels from the front to all cars on the train. And with sophisticated control, available, the engineer at the front of the train can, if skilled by learning from others and practice, individually control both brake and power of the helper, and thus have some control over slack action, very valuable for hill-and-dale railroading.
Then it is not a manned helper. It is a distributed power unit under the control of the engineer on the lead engine.
Just to be clear, on DP remote consists, independent control of braking is limited to dynamic braking. You can independently have the head end in dynamics and the DP consists in power. You can’t have the head end in power and the DP consists in dynamics. Air brake operation however is always in sync with the head end. You can’t set or release air on the DP remotes alone.
Jeff