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Railroads made great strides during the early part of the 20th Century in adding automatic block signal systems to their lines to improve safety. The Southern Pacific was one of the leaders in installing automatic block signals. The first automatic block signals on the West Coast were installed on the SP in 1904 between Oakland and Port Costa. SP only had automatic block signals on 3% of its main lines in 1902 (102 miles). By 1912 it had 75%, over 3,000 miles. In 1907 the US railroads installed automatic block signals on 2,005 miles of single track and 1,424 miles of two or more main tracks. The top company was the SP which installed automatic block signals on 991 miles of single track and 87 miles of double track that year. Second on the list was the UP with 431 miles of single track and 191 miles of double track. Subsidiary lines OSL installed 301 miles of automatic block signals and OR&N with 118 miles. Between 1905 and 1914 the US railroads had been installing an average of 2,448 miles of automatic block signals per year. This dropped to 1,748 miles per year during the 1915 to 1918 period.
This is a list of the number of miles (single or double track) protected by automatic block signals and the type of signals used on some of the major railroads in the West as of January 1, 1931. Total is the total mileage for the railroad: mainlines and branches.
SSW had just 1.6 miles of color light signals.