As a part of the original construction of any improvement herein authorized, or afterward as an alteration or renewal thereof, any such city, notwithstanding any charter provision to the contrary, may, at its own cost, construct, maintain and operate street railway tracks in the roadway thereof, and may provide electric power for the propulsion of cars, and may lease the use of such tracks and power for the operation of streetcars or interurban railways; or such city may authorize any operator of the street or interurban railways to construct and furnish such street railway tracks and electric power and use the same for street or interurban purposes, under lease or franchise ordinance: PROVIDED, That no such lease or franchise shall be exclusive, but shall at all times reserve the right to the city to permit other lines of street or interurban railway to use such street railway tracks in common with any preceding lessee or grantee, upon equal terms. The rate of lease or use of such street railway tracks for streets or interurban cars shall be as fixed by the legislative authority of the city, but shall not be less than one mill for each passenger carried, or ten cents for each freight car moved over such improvement. The income from such charges, rental and leasing shall be used wholly for the maintenance, repair and betterment of said improvement and the extinguishment of any debt incurred by the city in constructing it.