The Tunnels of the Inter-Vault Rail System

Background
A door in the Overseer's chamber, believed to lead out of the Vault, led to an underground rail network which seemed to connect a number of Vaults running parallel to the Cascade Mountain range. The train cars itself were destroyed and the tracks disabled, so travel by foot was the only option

Synopsis
A number of monstrous insects were encountered in these tunnels such as giant praying mantises, bombardier beetles, ticks, flies, and lion ants. The mantises had a tendency to reach out and strike at distance, as well as making grapple attacks. The flies were relatively harmless but upon death released a number of dangerous larva. The bombardier beetle would eject burning chemicals from its abdomen. The ticks attempted an ambush by dropping from the ceiling of tunnels. Lion ants lay in wait at the bottom of quicksand-esque pit, throwing debris to try and knock prey down to their waiting jaws.