Car float

A railroad car float in the Upper New York Bay, 1919. A tugboat (towboat) stack is visible behind the middle car.
1912 PRR map showing the Greenville Terminal and its car float operations, also the current crossing

A railroad car float or rail barge is a specialised form of lighter with railway tracks mounted on its deck used to move rolling stock across water obstacles, or to locations they could not otherwise go. An unpowered barge, it is towed by a tugboat or pushed by a towboat.

This is distinguished from a train ferry, which is self-powered.