Flying through Frankfurt from Dublin into Athens, so all within EU, looking to see if a cousin can meet us in the pre-security area to catch up and grab a quick bite. Do you think just under 5 hours will give us enough time to do that and get back through security for our flight? This will be in December.
The S-Bahn commuter trains (S8 and S9 lines) go right into Terminal 1. At the airport. It's about 12 minutes to the Hauptbahnhof (central rail station). You'd have plenty of time for lunch.
Just under 5 hours is plenty of time, especially if you're meeting at the airport. We once took the train into Frankfurt for an early morning walk and made it back when we had a 5 hour layover. The Frankfurt airport is big.
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