There is an article in the March, 2009 IEEE Spectrum (magazine for electrical engineers) about electrically controlled air brakes for freight trains. The author went with the engine crew on a loaded Norfolk Southern train at Bluefield, on the Pocohontas route. The article includes an interesting description of the braking of the train using a combination of dynamic and air brakes on the grades.
The link is http://www.spectrum.ieee.org/mar09/trainbraking
Sid Penstone