Said municipal corporations are hereby empowered and authorized to lease said tracks and equipment to other persons or corporations authorized by law to maintain and operate street passenger railways, upon such terms and conditions as may be agreed upon: Provided, however, That the amount of the annual rental to be paid therefor shall in no case be less than an amount sufficient to pay all annual maintenance and renewal charges on account of said construction, and, in addition thereto, not less than six per centum per annum interest upon the cost of the construction of said tracks and equipment.