Most of the EOG cars being used these days are High capacity power cars. Only one of these power cars can power an entire rake, though maximum efficiency is reached at 12-15 coaches. So usually, only one EOG is run while the train is running in one direction, and the other EOG in used in the opposite direction. For longer rakes, both may be run simultaneously for better efficiency. For short rakes like Shatabdi, usually only one EOG is sufficient. The other EOG is added to the rake as a spare.