At the risk of going overboard on this topic, here are some links with more information, photos and diagrams on the Seatrain car ferries. The film is especially interesting.
Port of Hemet, CA
Popular Science Article, 1929
Popular Mechanics Articel, 1947
Film: Seatrain New Orleans
Film made in 1929. A lot of footage showing cars being moved inside the ship.
Seatrain ferry loading 1956
The specialist crane hoist the cradle loaded with an ATSF boxcar from the Seatrain Louisiana
Seatrain Loader, Belle Chasse, LA