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 . 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. On the causeway is a signal box. The rail line is not electrified making the use of diesel locomotives necessary. Trains coming from origins further south like Hamburg change from an electric locomotive to a diesel locomotive at Itzehoe. Every day, more than 100 trains pass over the causeway, 50 of those ferrying cars (there is no road link to Sylt). Each year, the railway ferries more than 450,000 vehicles over the causeway. The causeway, which bears the Weimar Republic Reichspräsident Paul von Hindenburg's name, has interrupted the tidal flow, which until the causeway's appearance had flowed freely between Sylt and the mainland. This change in tides, it is believed, is part of what has led to the loss of a certain amount of land at Sylt's southern end. The causeway lies in the specially protected Zone I of the Schleswig-Holsteinisches Wattenmeer National Park. Walks on the tidal flats are not allowed here, although they are quite popular elsewhere. Situation before the causeway was built After the Second War of Schleswig in 1864, when Prussia took over Schleswig from Denmark, Sylt and Westerland belonged to the new Tondern district. The seaside bathing town of Westerland gradually grew in popularity. The west coast railway already ran from Altona by way of Husum and Niebüll to Tønder (then also in Germany and called Tondern). From here, the tracks were extended to the port at the Hoyerschleuse, whence paddlesteamers ran to Munkmarsch harbour on Sylt. The connection was at the tide's mercy, and in winter, the ice in the Wadden Sea formed an impenetrable barrier. Already being planned at that time was a rail causeway