Congress reserves the right at any time to alter, amend, or repeal sections 687a, 687a-2 to 687a-5,¹ and 942-1 to 942-9 of this title and sections 607a and 615a of title 16 or any part thereof; and the right of way herein authorized shall not be assigned or transferred in any form whatever prior to the construction and completion of at least one-fourth of the proposed mileage of such railroad, wagon road, or tramway, as indicated by the map of definite location, except by mortgages or other liens that may be given or secured thereon to aid in the construction thereof: Provided, That where within ninety days after May 14, 1898, proof is made to the satisfaction of the Secretary of the Interior that actual surveys, evidenced by designated monuments, were made, and the line of a railroad, wagon road, or tramway located thereby, or that actual construction was commenced on the line of any railroad, wagon road, or tramway, prior to January 21, 1898, the rights to inure hereunder shall, if the terms of said sections are complied with as to such railroad, wagon road, or tramway, relate back to the date when such survey or construction was commenced; and in all conflicts relative to the right-of-way or other privilege of said sections the person, company, or corporation having been first in time in actual survey or construction, as the case may be, shall be deemed first in right.