The Missouri Pacific Railroad , commonly abbreviated MoPac, was one of the first railroads in the United States west of the Mississippi River. MoPac was a Class I railroad growing from dozens of predecessors and mergers, including the St. Louis, Iron Mountain and Southern Railway (SLIMS), Texas and Pacific Railway (TP), Chicago and Eastern Illinois Railroad (C&EI), St. Louis, Brownsville and Mexico Railway (SLBM), Kansas, Oklahoma and Gulf Railway (KO&G), Midland Valley Railroad (MV), San Antonio, Uvalde and Gulf Railroad (SAU&G), Gulf Coast Lines (GC), International-Great Northern Railroad (IGN), New Orleans, Texas and Mexico Railway (NOTM), Missouri-Illinois Railroad (MI), as well as the small Central Branch Railway (an early predecessor of MP in Kansas and south central Nebraska), and joint ventures such as the Alton and Southern Railroad (AS).