The collapsed car tunnel is part of the underground tunnel system in the Capital Wasteland.
It's directly below Dupont Circle, and 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.