The A&P's earliest predecessor was the Pacific Railroad, incorporated by the Missouri General Assembly in 1849 to connect St. Louis and a point south of Kansas City across the center of the state.

After it defaulted on bonds that had been issued for the branch, the state seized the road from Franklin to Rolla and incomplete roadbed to Arlington in March 1866.

The property was sold in June for $1.3 million to explorer and politician John C. Frémont, who reorganized it as the Southwest Pacific Railroad in September.

The same conditions were applied to the Southern Pacific Railroad of California, which could build a branch to connect to the A&P near the eastern border of that state.

That company defaulted on its payments, and the state of Missouri seized the property in June 1867, selling it to a new South Pacific Railroad in July 1868.

[2] But this incarnation had similar financial problems; its Missouri division (Franklin to Seneca) was placed under receivership in November 1875, and the Pacific Railroad lease was canceled.

[2] In January 1880, the SL&SF came to an agreement with the Atchison, Topeka and Santa Fe Railroad, which had recently entered New Mexico from the north, whereby the two companies would jointly control the A&P.

[2] Through the Santa Fe Pacific, the AT&SF acquired trackage rights in January 1899 over the SP's Tehachapi Pass line, giving it access to the Central Valley of California and San Francisco Bay Area.