Update: Car Rental Return at Frankfurt Hauptbahnhof

If you need to return a rental car to the Hauptbahnhof (main train station) in Frankfurt, please note that the return location is now on the 6th through 8th levels at the garage at Stuttgarter Str. 35. There is NO SIGN at the garage entrance indicating you should return a rental car there. The solid gate will be down until you push a button and take a ticket from the machine at the entrance. After the gate rises, then you will see signs posted inside about car rental return. In theory, an agent may be there to take the car key from you. In fact, I had to go back inside the train station and return it to an agent in the rental car office next to the DB Reisezentrum. I hope this information will save others some time-wasting and stress.