Getting to Sharks by Public Transportation
Others have reported that to reach Sharks by public transport, Darmstadt Nordbahnhof is closer. I would like to add a little more useful information:
As you get out of the Darmstadt Nord station (there is only one way to get out) , you need to make a left turn and walk down FrankfurterStrasse (the station entrance is roughly in the middle of a rather long block and if you make a right turn, you will have wasted time and energy before you learn that you are heading in the wrong direction). At the next corner, the street sign is Martin-Luther-King-Ring (assuming you haven't crossed the street to the other side of FrankfurterStrasse). You then make a right turn and cross FrankfurterStrasse. The street name changes to Carl-Schenck-Ring once you cross the intersection. You then walk for about 5 to 6 minutes depending on your pace and then make a right turn onto Otto-Roehm-Strasse. If you want to take a short cut, you can cut across the shopping strip by taking a worn footpath across the lawn of the shopping strip before the intersection and then walk across the parking lot to Otto-Roehm-Strasse. You need to cross the street to the other side of Otto-Roehm-Strasse and continue walking for 2 to 3 minutes. There are really no FKK-Sharks signs until you're almost there and it is right by the curb outside of an auto dealership. Sharks is way in the back behind the dealership. The total trip walking from train station to the club is about 10 to 12 minutes for me.
The train ride from Frankfurt Hauptbahnhof to Darmstadt Nord is as short as 18 minutes depending on the schedule and the trains run very frequently. It is Euro 7. 3 one-way. If you do an all-nighter before flying out of Frankfurt airport, your fare is still Euro 7. 3 from Darmstadt Nord to the airport. The earliest ride is at around 4:35am from outside Darmstadt Nord. You will need to take the tram to Willy-Brandt-Platz and then switch to a bus which then takes you to Darmstadt Hauptbahnhof. From there, you take the train back to Frankfurt Hauptbahnhof and then switch to S8 or S9 to the airport. Rmv. De (and bahn. De) will give you all of the information you need.
You can store your luggage in the locker at Frankfurt Hauptbahnhof, it is Euro 3 for a small locker and Euro 5 for a large one if you use the lockers on the far right side (looking out from inside McDonald's for instance) of the train station alongside the last train tracks (I think it is Euro 4 and 6 for those lockers inside the building).