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.
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.
A nice chain reaction can be set off here by lighting one of the wrecked buses.