Public Money and Private Power
The scandal was not simply about railroad investors making a lot of money.
Crédit Mobilier became a larger political problem because the same insiders could influence both sides of construction contracts, federal support helped make those contracts possible, and stock transactions brought the conflict into Congress.
Taken together, those connections blurred the line between private investment, public money, and political power during Reconstruction. The scandal showed both the possibilities and the risks created when the expanded federal role that grew out of the Civil War depended on private corporations to carry out national projects.

Sources: Wilson Report, 1873; Poland Report, 1873; Richard White, Railroaded; Paul Kens; Mark Wahlgren Summers.