On reserve you might get called out to BWI or DCA, but vast majority of flying is out of IAD. All pairings start and end at same airport, so I've never had to use ground transport between, just commuted in. There are some other threads with detailed train/bus/shuttle options between (silver line may actually open someday)

Similar situations exist in other domiciles as well (e.g. LAX-SNA-ONT, SFO-SJC, EWR-LGA)