Though I've sat in one of those parts of BNA where they squish a whole bunch of gates together and overheard Southwest employees working their way through a whole bunch of equipment swaps in the name of getting the largest number of flights back to schedule as closely as possible after some modest delays that were threatening to cascade. (And WN's all 737 approach makes it quite easy to start shifting aircraft around compared to other airlines that can't match the scheduled mix of aircraft )