A train ferry is a ship designed to carry railway vehicles. Typically, one level of the ship is fitted with railway tracks, and the vessel has a door at the front and/or rear to give access to the wharves. In the United States, train ferries are referred to as "car ferries". The wharf (sometimes called a "slip") has a ramp, linkspan or "apron", balanced by weights, that connects the railway proper to the ship, allowing for the water level to rise and fall with the tides. For an example of a specialized slip to receive railcars see ferry slip.
While railway vehicles can be and are shipped on the decks or in the holds of ordinary ships, purpose-built train ferries are much quicker to load and unload, especially as several vehicles can be loaded or unloaded at once. A train ferry that is a barge is called a car float.
Burlington Northern might or might not own a few of these. But given the investment Buffett is putting into railways, one can't help wondering why some rail ferry owners are selling at a discount.