The Crédit Mobilier scandal of 1867, which came to public attention in 1872, involved the Union Pacific Rail Road and the Crédit Mobilier of America construction company in the building of the eastern portion of the First Transcontinental Railroad
In simpler words, it was a company that overcharged Union Pacific on purpose so that stockholders could pocket the different and give some of the money to Congressmen. A scandal, of course.
