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The railroad was built with rail weighing 33 kilograms per metre (67 lb/yd) and required more than 3000 bridges. There were 126 tunnels in the Zagros mountains, the longest being 2.4 kilometres (1.5 mi).Grades averaged 1.5 percent south of capital, but then increased to 2.8 percent to cross the 2,220-metre (7,270 ft) pass between Tehran and the Caspian Sea.
