There are two basic flows of rail traffic for the automotive industry: automotive parts from suppliers, internal or external to the main automobile manufacturing companies, to automobile assembly plants; and finished automobiles from assembly plants to distribution centers around the country/ies. Although these traffic categories have quite different requirements, they may often be carried on the same dedicated trains when parts for some vehicle types travel from a main focal point of the industry to regions having both assembly plants for those vehicle types and distribution centers for others assembled at the main focal point..
There were Auto Parts Box Cars, automobiles carried in regular box cars and in experimental specialized cars, and automobiles carried on regular flat cars in the 1940s and 1950s, but the volume of traffic seems not to have warranted their carriage in special (unit) trains outside of general merchandise service. In part, this was because the production of all types of vehicles was distributed among assembly plants located near the main market centers for these vehicles, with their own groups of parts suppliers, and thus only major specialized parts needed to be shipped long distances.
Most such traffic originated in any volume with New York Central, Grand Trunk Western, Wabash, or Detroit, Toledo & Ironton in the Detroit area.
With the centralization of vehicle model production (but not necessarily in Detroit), the vast increases in vehicle imports (particularly at west coast ports), and the development of multi-level autorack cars, transport of finished vehicles in dedicated trains from assembly plants to market distribution centers became commonplace. Operationally, dedicated autorack trains behave much like intermodal trains, running through intermediate terminals, except for thousand-mile inspections and on-line fueling of locomotives, conducted at the same place as for intermodal traffic.
With the concurrent development of just-in-time manufacturing, dedicated trains of auto parts for assembly plants not located in the Detroit area also became common. The lines on which these trains originate in the Detroit area remain the same, only the names of the railroads have changed!
BNSF runs dedicated autorack trains from Argentine Yard (Kansas City) west on the Transcon to San Bernardino in Southern California.
Union Pacific runs dedicated autorack trains west from Armourdale Yard in Kansas City, over the Marysville sub. to Gibbon Junction and then the Overland Route (and perhaps the former Western Pacific) to the San Francisco Bay Area, and southwest (since the SP merger) on the Golden State Route and Sunset Route west of El Paso to Mira Loma in Southern California. (Prior to the SP merger, these trains went via the Overland Route to Ogden and then the Salt Lake Route to Southern California.)
As the successor to the Grand Trunk Western (a former subsidiary), CN runs many dedicated automotive traffic trains between Detroit and Chicago for forwarding to other railroads.
CSX continues to handle the former Conrail Multi-Level (ML) fleet of trains, east from the Detroit area, over the Water-level Route east of Berea, serving New England and New York through Selkirk Yard.
NS now owns the old Wabash line from Detroit to Kansas City, connecting the automotive plants in Detroit with those in Kansas City, and with the runs west on other railroads from Argentine (BNSF) and Armourdale (UP) Yards.