Originally Posted by
SandBoxJohn
@ djp98374
The Primary reason for the closing south of National Airport is to realign the existing mainline tracks and connect them to the tracks that pass through the new Potomac Yard station. As part of this project a double crossover is being installed north of the new station just south of where the existing alignment connect to the new alignment. Ground work for the northwest corner of the new double crossover is partly under the existing inbound mainline track C1.
There's a photo of this double crossover, apparently brought to the construction site preassembled, on the
City of Alexandria's page about station construction. The other good way to get a peek at the progress: have a window seat on the left side of the plane as you land at DCA from the south.