Correspondence, journals, board minutes, reports, dispatches, financial records, printed matter, maps, and other papers relating primarily to Plumb's diplomatic service in Mexico and to negotiations with the Mexican government on behalf of the United States and American investors in the Mexican railway system. Includes reports and dispatches to U.S. secretary of state William Henry Seward regarding diplomatic and political affairs in Mexico. Other subjects include steam communications, railroads, and cotton manufacturing in the American West and Mexico; the Mexican International Railroad and a proposed Tehuantepec canal and railway; Mexican insurrectionists including Porfirio Díaz; the Mexican War; and conditions in the southern states in the years following the Civil War. Also includes an account of an 1849 voyage from New York to Rio de Janeiro and then San Francisco by way of Cape Horn. Correspondents include Nathaniel Prentiss Banks, Lewis D. Campbell, Robert S. Chew, R.S. Chilton, Henry Clay, Edgar Conkling,Thomas C. Cox, Caleb Cushing, Charles A. Dana, William Pitt Fessenden, Hamilton Fish, Baron von Geralt, Robert Grant, David Hoadley, William Hunter, Benito Juárez, Matías Romero, Charles Sumner, J. Edgar Thomson, Sebastián Lerdo de Tejada, Manuel Murillo Toro, and C. Lennox Wyke.