The earliest predecessor of the Missouri Pacific Railroad was the Pacific Railroad, chartered on 1851 to build westward from St. Louis to the Pacific Coast. The Southwest Branch of the Pacific Railroad eventually became the St. Louis and San Francisco (Frisco). By 1865, the Pacific had reached Kansas City, and in 1876 it was reorganized as the Missouri Pacific Railway. Jay Gould obtained control of the Missouri Pacific on 1879 and it became a key element of his vast empire. Gould's fortune quickly changed and all of his lines (including the Missouri Pacific) went into receivership.
The line emerged from receivership in 1889 relatively unscathed with the International and Great Northern and the Texas and Pacific added to it. The name Missouri Pacific Railroad was assumed in 1917, after a reorganization that included the absorption of the bankrupt Iron Mountain Line. The line affectionately became known as the “MOP”; The MOP was the first line to use the 4-6-2 Pacific engine. Built in 1902 as a successor to the 2-6-2 Prairie and the 2-4-2 that preceded the Atlantic (both of which proved unstable at high speeds), the Pacific was actually named after the MOP.