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

At the ArchivesShow

Go there.Show

Inside the metroShow

On the platformShow

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.
Overhead mapShow

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.
Standing at the cornerShow

Looking at Kings controlled territoryShow
