The book's beginning details how the southwestern territory those rails crossed was purchased from Mexico in 1854.
James Gadsden of Charleston, South Carolina started championing a southern cross-country railroad in 1846 to improve his hometown's economy and to export slavery west of Texas. At the conclusion of the Mexican American War two years later, the United States obtained 600,000 square miles of new territory, but not enough to accommodate the southern route. That is why, at the urging of Jefferson Davis, Gadsden was appointed Minister to Mexico.
Gadsden's negotiations to acquire more Mexican land were complicated by several factors, including dubious instructions from a secret messenger. He was able, however, to finalize a treaty, which was later substantially altered by the United States Senate, that resulted in the Gadsden Purchase.