In 1943, when the Southern Pacific Railroad placed an order for fourteen new "Daylight" locomotives from Lima, World War II was raging, and the War Production Board dictated what types of locomotives could be manufactured. SP's order was turned down, with the government declaring that streamlined passenger engines were not necessary and would not help in moving wartime traffic. Southern Pacific re-designed the engines for general service and it was finally approved. The smaller and power-starved Western Pacific Railroad was also looking for locomotives at the time and had first requested diesels, then a different style of steam locomotive. The War Production Board instead diverted six GS-6s (reportedly to have been numbered SP 4470-4475) to the WP.
The GS-6s were used by Southern Pacific for the San Joaquin Daylight, as well as San Jose-San Francisco commuter trains and freight service.
The Western Pacific used its GS-6s (GS-64s as WP classified them) on passenger trains and in freight service as well. They acquired a different look from their SP sisters when the WP applied the "elephant ear" style smoke deflectors to all six locomotives. When the Western Pacific dieselized in 1953, they sold three GS-64 engines (WP 481, 484 and 485) to the Southern Pacific for spare parts, but kept the tenders and converted them to steam generators for rotary snowplows.
Between 1953 and 1958, all but one of the GS-6s were all retired and sold for scrap when they were replaced by EMD E Series and ALCO PA Series diesel locomotives, while the F Series units were used as "spare units" when they were all first ordered.
The only surviving GS-6 is No. 4460, which pulled the final movement of steam on the Southern Pacific Railroad in 1958 and seen on static display at the Museum of Transportation in St. Louis, Missouri.
- The tender of Western Pacific GS-6 #484, which was used in its final years as a water and fuel tank for a rotary snowplow, is stored at the Western Pacific Railroad Museum in Portola, California.
- The GS-6s are the "Cousins" to the GS-4s, and GS-5s.
- They looked similar to the GS-2 with a silver smokebox with a cone-shaped single headlight casing and 73-inch (1.854 m) drivers. Like all GS engines they had teardrop classification lights and an air horn to supplement their whistle.
- The GS-6 lacked side skirting and red and orange "Daylight" paint found on previous locomotives of the GS class, and were painted black and silver instead.
- They were built during World War II, but was never painted the famous Daylight paint scheme but Instead, it was painted black and silver, thus giving it the nicknames "War Baby", "Black Daylight".
- When six of the locomotives were sent to the Western Pacific, they were given "elephant ear" style smoke deflectors.