McClellan resigned his commission January 16, 1857, and, capitalizing on his experience with railroad assessment, became chief engineer and vice president of the Illinois Central Railroad, and then president of the Ohio and Mississippi Railroad in 1860.
McClellan tog sin afsked fra hæren den 16. januar 1857 og udnyttede sine erfaringer med vurdering af jernbaner og blev overingeniør og vicepræsident i Illinois Central Railroad og derpå præsident for Ohio and Mississippi Railroad i 1860.