For 10 points, name this project that, when first completed, connected Omaha to Sacramento by one set of tracks.
The Welsh engineer John Plumbe held the first convention advocating for completing this type of structure. A bill was submitted to Congress with the goal of creating one of these by Asa Whitney. The first of these structures was completed by Thomas C. Durant and Leland Stanford, the latter of whom drove in the (*) golden "last spike" in 1869 in Promontory Point, Utah. That spike joined the Central and Union Pacific to create this type of structure. For 10 points, name this project that, when first completed, connected Omaha to Sacramento by one set of tracks.
The Welsh engineer John Plumbe held the first convention advocating for completing this type of structure.
{transcontinental} railroads [or equivalent answers; prompt on partial answer]