- "A Good Will Relief transport got stuck in this tunnel when a transport of chemical waste was derailed, covering one end of the tunnel in a deadly toxic cloud. Soon after, the military declared it a quarantine zone and blocked any passage in and out of this place. Like all tunnels, it is crawling with enemies, but the stuff left there is just too good to leave to rot. Got to be careful. Some survivors might have tried to get their hands on the supplies, who knows what happened to them?"
- —Loading Screen
It is a large underground rail tunnel which contains two crashed trains. One end of the tunnel is inaccessible due to a relatively large chemical spillage which has created a large deadly cloud of gas, which will rapidly kill you if you enter it. Also, a large amount of viral zombies are present, so extreme caution is recommended when in this quarantine zone. The zone itself contains 5 DROP packages and a number of GRE crates.
The first supply package can be immediately found in the searchable train car near where the player first entered. The rest however, are located further in the tunnel where the infected awaits. Most of the horde consists of biters but a handful of bombers also occupy the area. Should they explode, the sound will attract virals.
The last supply package is not found in any of the train cars but at the end of a narrow hallway filled with the infected. Virals will keep spawning at this end and will often rush at the player.
Most, if not all, of the GRE crates are in areas where virals can rush at and attack players. One of them is even placed nearby a goon and close to the toxic chemical spill where virals tend to gather.
- It is possible to access Bright Mountain Tunnel during the story mission The Pit after leaving the arena before the last fainting animation takes place. Although player's movements will be limited to walking, all purchased skills will still work, which allows to complete the quarantine zone through the use of camouflage, takedowns and weapons found in GRE crates (weapons from arena can be scrapped to create lockpicks).