As a follow-up on my prior post about Philadelphia, allow me to present post offices inside train stations. Two of them, in Philadelphia, are in major transit hubs, and when I visited them, were being well used:
In Philadelphia, the 30th Street Station is the major Amtrak hub, though it also serves the SEPTA [Metro Philadelphia] system. Penn Center is a major subway station in downtown Philadelphia.
Below, two post offices in train stations that effectively constitute the centers of their respective towns.