Head out the back door of GNR and take the collapsed car tunnel to the other side of Dupont. Then take the metro tunnels until you're there. Hey, be careful though... you'll have to find another way back 'cause it's a one-way trip.”— Three Dog
The collapsed car tunnel is part of the underground tunnel system in the Capital Wasteland.
It's directly below Dupont Circle, to which it has two exits, as well as Dupont Circle station in the Metro system. The tunnel is poorly lit with only several fires to provide light for the Lone Wanderer. The entire tunnel is filled with various feral ghouls, the amount of each and type depending on the Lone Wanderer's level.
This tunnel contains feral ghouls and the odd raider group.
The collapsed car tunnel appears only in Fallout 3.