In Carthage are a number of train stations. For the archaeological site and the amphitheater you get off at Carthage Hannibal. The old harbor is located at station Carthage Byrsa.
A train ride from La Goulette (Le Bac or Goulette Vieille) takes less than half an hour. For a ticket you pay TND 0.50 per person (one way). The trains run frequently (4-5 per hour). You can check the train timetable in advance.
How do I get to the train station from the cruise port in La Goulette?
Le Bac train station is located across the parking lot opposite the exit of the modern cruise terminal. Goulette Vielle train station is situated a little further from the cruise terminal northeast of the docking area.