You have some valid points. Train brakes are not mean't to be applied and released over and over as the reservoirs can deplete and not provide adequate pressure for a solid brake application if needed. Train brakes are actually designed to operated at much higher temperatures so '"riding" the brakes down a grade is normal. Engineers can modulate the pressure to fine tune the application depending on where they are, minimize air consumption and keep the slack out. That is a reason why "knowing" the territory can be so critical. After an emergency application it can take several minutes for the system to recover and pump off the brakes, especially on these longer trains so common today. Each individual car's reservoir has to recover. I think it takes about 90 PSI throughout the train to release all the brakes,
The reason dynamic brakes are so great is they take some of the pressure (no pun intended) off the air brakes. A 5-10% reduction in air brake line pressure can be kept steady by modulating the dynamic brakes as grade vary. Grades for trains are much smoother over long distances where truck brakes might get a chance to cool (a bit) going more up and down shorter grades whereas train brake applications can last for many miles.
As I understand it full air braking releases the dynamic brakes for reasons I don t really understand.
