Sorry, could you please clarify that? It seems that the first mako reactor, the train, the sector 7 slums, the second train, and the pillar are all separate and distinct places in which Jessie, Wedge and Biggs are to be found.

I'm fairly sure you mean: "Where else do they pop up in the game after the Pillar?", but I'd just like you to clarify that if you don't mind please. It would be a shame to lose at this point due to a misunderstanding.