Western Pacific Railroad 6424 was a short mill gondola built in September 1953 as part of a group of 100 cars purchased when the Western Pacific needed to expand steel coil service for a U.S. Steel mill in Pittsburg, California, served by its Sacramento Northern Railway subsidiary. The design was unique to the railroad, because heavy steel coils overloaded typical 45- to 50-foot gondolas before they were full, so these cars were made very short to reduce wasted space. Its greatest significance was technological: from the earliest days of railroading, axle bearings had commonly been plain or friction bearings using oil waste journals, which were vulnerable to wear, low oil, loss of waste, and destructive overheating known as a hot box. Beginning in the 1920s, the Timken Company promoted roller bearings, which required little lubrication and reduced friction, but railroads were slow to adopt them widely for freight cars. These cars became the first large production batch of freight cars built with sealed roller-bearing trucks, placing Western Pacific at the forefront of a major railroad technology shift that eventually made plain bearings illegal in regular rail service.