On February 1, 1832, The Western Immigrant, an Ann Arbor newspaper, printed the first published proposal for a transcontinental railroad.
Source: Michigan History
Or was it February 6, 1832? See John Debo Galloway, The Transcontinental Railroad, Chapter 3, Simmons-Boardman, New York, 1950.
For a related article, see “Dr. Hartwell Carver’s Proposal to Build a Railroad from Lake Michigan to the Pacific Ocean”
Asa Whitney Pitches Transcontinental Railroad to Congress, from Lake Michigan to the Pacific, January 17, 1848.