United has a much bigger footprint in the DC area. NB as a local can be super easy and probably better on reserve than being a junior lineholder. The bus isn’t like what Otterbox said above. The difference between it and the 737 in terms of trips out of DC is negligible in terms of trip construction. The 737 actually has more 4 leg days than the Airbus, but on both fleets they are pretty rare.
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