Thomas C. Durant (1820-1885) was perhaps the most colorful and least liked of all the men involved in building the transcontinental railroad. Durant had been educated in medicine (and was often called the Doctor or "Doc"), but he made his fortune as a prodigious schemer. His first big score had been smuggling contraband cotton from the Confederacy during the Civil War, but when he became involved in railroad building, Durant hatched a scam of unprecedented proportions.
Using his investment in the newly incorporated Union Pacific, Durant established himself as the line's vice president and general manager. Then, he secured the Union Pacific's construction contract for the Crédit Mobilier of America, a company controlled by none other than Durant himself. The Doctor then ran up construction costs for the line and awarded himself (and fellow Crédit Mobilier investors) handsome payments while nearly bankrupting the UP.
The other top men of the Union Pacific probably counted themselves lucky when they managed to force Durant's resignation shortly after the line was complete. As it turned out, he made off with a fortune to scheme another day, and they were left to answer for the Crédit Mobilier scam in 1872, when it became the biggest financial scandal of the century.