Each and every grant of right of way and station grounds made prior to February 25, 1909, to any railroad corporation under sections 934 to 939 of this title, where such railroad had not been constructed and the period of five years next following the location of said road, or any section thereof, had on that date expired, is declared forfeited to the United States, to the extent of any portion of such located line then remaining unconstructed, and the United States resumes the full title to the lands covered thereby free and discharged from such easement, and the forfeiture declared shall, without need of further assurance or conveyance, inure to the benefit of any owner or owners of land conveyed by the United States prior to such date subject to any such grant of right of way or station grounds: Provided, That no right of way on which construction was progressing in good faith on February 25, 1909, shall be in any wise affected, validated, or invalidated, by the provisions of this section.