I worked at Buick for 40 years...beginning in 1967. The bodies were always trucked across town We had a section of the assembly plant(s) (there were three at that time) called "body receiving" The semi trailers were specially built, so that when they backed into the dock, the bodies set on cradles that were lined up with a conveyor system, and it was easy off with the new and back on the trailers with the empties. Maybe at one time they were shipped by rail way-way back, but I never heard of it happening. The Riviera Bodies were built in Euclid Ohio, and shipped all the way to Flint via semi also. In 1995, Riviera production moved from Flint to Lake Orion assembly.