The longest train bridge in the US In order to cross the Mississippi River, while enabling large vessels to pass underneath, trains begin a long, slow climb from the near sea-level swamps of southern Louisiana, two miles away from the river itself. The structure that allows this is part of the longest railway bridge in the USA, named after, and commissioned by Mr. Long, the notorious Louisiana governor. Built in 1936, the bridge carries both automobiles and trains in and out of New Orleans. The thick sediment of the riverbed required the construction of a cement foundation extending 170 feet beneath the water and mud. Despite nearly 800 feet of horizontal clearance between the piers of its cantilever truss, it has been struck by ships several times.