The Hindenburgdamm or Hindenburg Dam is an 11 km-long causeway joining the North Frisian island of Sylt to mainland Schleswig-Holstein. Its coordinates are (54°53′N8°33′E). It was opened on 1 June 1927 and is exclusively a railway corridor. The companies that built the Hindenburgdamm, a job that took four years, were Philipp Holzmann AG of Frankfurt, working from the mainland, and Peter Fix Söhne of Duisburg working from Sylt. A train trip along the causeway takes about 10 minutes, and the time between the auto terminals at Niebüll on the mainland and Westerland on Sylt is about 30 minutes. The Hindenburgdamm is part of the railway line known as the Marschbahn ("Marsh Railway"), which is double-tracked along much of the route, although there as yet exists a single-tracked stretch.