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The EMD LWT12 was a power car built by General Motors Electro-Motive Division (EMD) in 1955, to pull a lightweight passenger train. Both components were jointly developed by GM under the project name, Train Y, but were later marketed as the Aerotrain (GM). Diesel power was provided by an EMD 567C 12-cylinder engine, which produced 1,200 hp (890 kW). Two other GM Diesel engines provided current for train-heating, lighting and air-conditioning. The LWT12 was underpowered, especially on grades, and the Santa Fe and Union Pacific Railroads were required to supply a "helper" unit to assist them in service. The LWT12 was essentially an EMD SW1200 switcher locomotive, suitably geared for high-speed passenger service (83 mph) and wrapped in a distinctive aerodynamic shell. Its industrial styling was inspired by the hoods and grills of futuristic automobiles then on GM's drawing boards.
Originally, the EMD LWT12 was intended to be part of an inseparable set along with ten specially designed high-speed, low-cost, 40-foot (12.19 m) passenger cars. These cars were built from bus bodies sourced from GM's GMC division which were then widened by 18 inches (457 mm), had their front and rear modified and were attached to a generic undercarriage. The advantages of this design were that instead of refurbishing the whole carriage, the body mounted on the undercarriage would be scrapped in whole and a complete new modified bus body would be installed in its place with all of the different technical advances that had been developed, essentially resulting in a completely new car for a fraction of the cost. Also, all parts used by these carriages were sourced internally by GM and were also used in other products. All of this meant that initial outlay, as well as maintenance costs, were significantly lower than traditional passenger cars resulting in a situation where railroad companies could offer rail fares similar to bus fares of the time. This design, as well as the EMD LWT12 were the cover feature article of the September 1955 Popular Mechanics magazine. Two of these whole train sets were built for the purpose of being driven across the United States for public viewing.
Only three LWT12 units were built. The first, EMD serial number 20826, entered service with the Chicago, Rock Island and Pacific Railroad to pull the Talgo "Jet Rocket" between Chicago and Peoria. The coaches for this train were made by American Car and Foundry. The Rock Island preferred the single-axle Talgo cars over the double-axle GM bus-body coaches being built for the Aerotrain. But they were also drawn to the futuristic styling of GM's locomotive, over the more traditional look of the Fairbanks Morse unit selected by ACF to pull the Talgo cars, so the railroad mated the two to form a unique lightweight consist of its own.
The second and third GM diesels, EMD serial numbers 21463 and 21464, powered the two GM Aerotrain demonstrators that toured the country in 1955, before being leased to four railroads for revenue service testing in 1956-57. All of the roads rejected the Aerotrain, and the two GM demonstrators were eventually sold at great discount to the Rock Island Line, where they join the Jet Rocket hybrid. Two of the three LWT12 locomotives continued in commuter service with the Aerotrain coaches, until retired in 1965. They are now preserved and on display at two US transportation museums.