This seemed to me like an early, less efficient predecessor to modern containerized shipping. The obvious disadvantages are that you have to carry the whole train cars, they don’t stack well and you can’t put them on a truck. It turns out that if you look at the origins section of the Wikipedia page for containerization, they mention this company, Seatrain Lines (which went bankrupt in 1981).

These ships could carry 95 train cars. For comparison a modern Ultra Large Container Vessel can carry 15,000 containers with a capacity similar to a train car.

Steamer Carries a Mile of Cars

Loaded Freight Train, Hoisted Aboard by a Mammoth Crane, Is Swallowed by Ocean Ferryboat

A LOADED train almost a mile long disappears into the hold of a monster ocean ferryboat, two thirds the size of the liner Mauretania, which recently began operating between New Orleans, La., and Havana, Cuba. The freight cars, hauled to the dock alongside the Seatrain, as the vessel is called, are picked up in cradles by a giant crane and lowered into the hold.