WMATA - Unique Distinction Globally
The Washington DC area's WMATA transit agency is the only agency IN THE WORLD where both of these criteria are met:
-Serves at least two airports with metro style rail operated by the same transit agency
-Neither airport serves as the terminus of the rail line serving it (at least one non-airport station on either side of the airport)
Many cities around the world qualify for serving two airports with one system, but WMATA sits apart once you require neither to be the end point of their rail line. The Blue and Yellow lines continue past DCA to several stations further south, and the Silver Line serves two stations further west in Loudoun County after IAD. You wouldn't expect DC/NOVA to be the only place where this happens.
DCA was really just a matter of geography. Thr airport is close to the city center and there are many populated communities further south. Dulles was a combination of political boundaries and smart investment. IAD sits 25 miles from downtown DC, and the Silver Line added 19 miles of new Metro service to reach Dulles. Planners were smart to basically say, were going to spend billions to get to Dulles, why not add some track after the airport (they built 4 more miles) to connect more communities and get a higher ROI on that huge investment? They also saw Loudoun County demand a pair of usable, developable stations if they were going to chip in financially, since the airport is only just over the border from Fairfax County.
Other cities should realize they stand to gain by extending lines beyond the airport, as you then add more communities to often long and expensive metro lines, you add pick up/drop off opportunities on the further end of the airport, thus reducing airport road traffic, and you can better locate end-of-line infrastructure like yards and pocket tracks away from the airport where it may be easier or operationally more efficient than in an airport terminal.
I find it astounding that only WMATA meets those two straightforward criteria. Some others are close, but fail on at least one point.