There is forfeited to the United States, and the United States resumes the title thereto, all lands granted prior to September 29, 1890, to any State or to any corporation to aid in the construction of a railroad opposite to and coterminous with the portion of any such railroad not on that date completed, and in operation, for the construction or benefit of which such lands were granted; and all such lands are declared to be a part of the public domain: Provided, That sections 904 to 907 of this title shall not be construed as forfeiting the right-of-way or station grounds of any railroad company granted prior to September 29, 1890.