So how do you get there?
By taking the train, of course! All you need to do is reach Union Station which is in the middle of the supermutant-infested DC ruins.
First, from The Mall make your way East towards the Capitol Building then North towards the National Archives.
Walking across The MallShow
Next, your destination is the Penn. Ave/The Mall Metro which is located behind the Archives.
At the ArchivesShow
Once inside the metro, continue straight down the hall and on to the elevated platform (mind the feral ghouls). Direct your gaze over the side of the platform to the right/East. You will notice that one of the train tunnels is free of rubble.
Inside the metroShow
On the platformShow
To continue on to the station, take the exit at the end of the tunnel to Judiciary Square Metro (yes, this is a new area) which will lead you to a new DC mall area. With a little bit of exploration you will find the station building.
Looking over the sideShow
Additionally, here is an overhead composite map of the layout of The Mall with the Penn. Ave./The Mall dungeon highlighted in yellow.
Disclaimer! Like most places in DC, the station can be a very challenging or downright dangerous place for low-level characters.
How do I get back to the Capital Wasteland?
Things didn't really work out for you. Getting shot in the head was a real wake up call. It's made you realize some things and now it's time to pay a visit to the homestead. Could it be that train you took from DC to New Vegas all those years ago is still running?
It sure is! That old train station is still in Freeside where you first got off it all those years ago. Maybe you don't remember where it is on account of recent events having to do with your noggin.
To get to the train, travel to Freeside North and head over to the Mormon Fort.
Continue past the Mormon Fort, down the street and under the overpass.
Standing at the cornerShow
Looking at Kings controlled territoryShow