The Underpass has little to do with the story itself, however it does provide the closest access point to Patricia Tannis, who is visited several times during the course of the main storyline. It also provides an Ammo Vendor and a Med Vendor for resupply and loot sales to adventurers in the area.
Directly north of the station is a tent with several small lockers and a box with cash in it. To the north-east are a few hanging or flying rakk.
There is a red weapons chest on a ledge above the outpost. This can be accessed by a circuitous path to the right of the Rust Commons East gate and those venturing up to it will be attacked by a flight of rakk.
Just west of the station (near the bridge) is a red weapons chest; this makes the station a good place to go for farming. A badass will spawn (level 25 in the first playthrough and level 45 in the second playthrough), so come prepared.