I don't have any GM related evidence, from my experience watching new vehicle deliveries as a kid - trains were used to transport new vehicles across the country to a rail yard. After that, trucks with car hauling trailers delivered new vehicles from the freight yard to the dealers.
Up until the mid/late 60's many new cars were transported by rail on open cars, complete with wheel covers ready for delivery to the customer(s). Somebody finally wised up to the amount of theft going on. Wheel covers and other accessories moved to the trunk and the rail cars converted to perforated sides to further discourage thieves.
