Have you thought about adding a cave complex and tunnel network to the lowest level of the GMS compound, where the raiders come from?
Maybe make it an additional quest line, helping to explore further for the South Rail and see if there are any bandits or anything worth taking.
You get the keycard from Gorsky, then report back to Tanner, and inform him of the raiders and an underground tunnel complex, and he tasks you with taking a look around to make sure that the threat is minimal. maybe you find a raider camp of some sort, small and unorganized, with the leader still around but the lieutenant, whom you killed, as missing. Maybe they're having problems of their own with some sort of creature or power supply or something, or they have something the Southside station could use, be it a power core or something else.