Union Pacific Railroad (UP) was incorporated in July 1897 as a reorganization of Union Pacific Railway, which was in receivership since 1893. Union Pacific Railway had been incorporated in January 1880 as a consolidation of the original Union Pacific Railway (chartered-in-1862), the Kansas Pacific Railway, and the Denver Pacific Railway & Telegraph Co. UP Railway connected with Central Pacific in May 1869 at Promontory, Utah. UP lines in Kansas and Colorado were purchased from predecessor companies.

Oregon Short Line Railroad (OSL) was incorporated in February 1897 as a reorganization of Oregon Short Line & Utah Northern Railway, in receivership since October 1893. Oregon Short Line & Utah Northern Railway had been incorporated on August 19, 1889 as a consolidation of the original 1881Oregon Short Line Railway and six other railroads operating in Utah and Idaho. The original OSL Railway began construction in May 1881 at a connection with UP at Granger, Wyo., and was completed across southern Idaho to Huntington, Ore. by November 1884. All OSL lines in Utah were purchased from predecessor companies.

Los Angeles & Salt Lake Railroad (LA&SL) was a name change in August 1916 from the original 1901 San Pedro, Los Angeles & Salt Lake Railroad. The SP, LA&SL (and the later LA&SL) were controlled by William Clark until 1903, then jointly controlled by Clark and OSL until April 1921, when UP bought Clark’s interest, making LA&SL jointly owned by UP and OSL, which itself was fully controlled by UP. The connection between Salt Lake City, Utah, and Los Angeles, Calif., was completed on May 26, 1905 at Erie, Nev.

Oregon Washington Railway & Navigation Co. (OWR&N) was incorporated in November 1910 as a consolidation of Oregon Railroad & Navigation Co. and 14 other companies in the states of Oregon and Washington. The Oregon Railroad & Navigation Co. was incorporated in 1897 as a reorganization of Oregon Railway & Navigation Co., which had been in receivership since 1893. The original Oregon Railway & Navigation Co. was organized in 1879 and completed a connection with OSL at Huntington, Ore., in November 1884. OR&N was operated by OSL from January 1887 to July 1889, then operated jointly by GN, NP, and OSL until July 1899, when OSL purchased full control of OR&N.

Union Pacific controlled the OSL from 1899; the LA&SL from 1921, and OWR&N’s predecessor OR&N from 1899. OSL, LA&SL, and OWR&N were formally leased for operation by UP in November 1935, and all four railroads were consolidated for single-line operation as the Union Pacific System on January 1, 1936. The corporations remained separate as wholly owned companies until December 1987, when LA&SL and OWR&N were merged into OSL on December 30th. OSL (and several other small companies) was formally merged into UP, on December 31, 1987.

Union Pacific was granted control of Missouri Pacific and Western Pacific on 22 December 1982. The merger application was filed on 15 September 1980; approved by the ICC on 20 October 1982, and cleared by the courts on 22 December 1982.
    Western Pacific was immediately merged into UP.
    Missouri Pacific was formally merged into UP on 1 January 1997.

Union Pacific was granted control of Missouri Kansas Texas on 10 August 1988.
    MKT was formally merged into Missouri Pacific on (?).

Union Pacific was granted control of Chicago & Northwestern on 12 April 1995.
    C&NW was formally merged into UP on (?).

Union Pacific was granted control of Southern Pacific on 11 September 1996. At the time, Southern Pacific also controlled Denver & Rio Grande Western (D&RGW) and St. Louis Southwestern (SSW, Cotton Belt).
    The D&RGW was formally merged into UP on 30 June 1997.
    SPCSL (Southern Pacific Chicago Service Lines??) was merged into UP on 30 June 1997.
    SSW was formally merged into UP on 30 September 1997.
    SP was formally merged into UP on 1 February 1998.

    SP merged with D&RGW on 12 September 1988.