It shall be lawful for, and the right is hereby conferred upon, cities of the second class of this Commonwealth, to sell, pledge, or lease transit facilities, or any one or more of them, and to grant licenses for the use of transit facilities, or any one or more of them, or any part thereof, to any corporation or corporations duly authorized and empowered under the laws of this Commonwealth to use or operate the same; also to enter into agreements for the construction or operation, or for both the construction and operation, of such transit facilities, or any one or more of them, by any such corporation or corporations; upon such terms and conditions, including the prescribing and fixing of rates for transportation, as the corporate authorities of such cities shall determine: Provided, No such lease, license or operating agreement shall be for any period longer than fifty (50) years: Providing further, That no such agreement for the construction of such transit facilities, or any one or more of them, or any part thereof, shall be made or entered into unless the same provides that work on such construction shall begin within eighteen months from the date of said agreement, and be completed within a fixed time designated in said agreement: Providing further, That no agreement for the leasing, licensing, using, or operating transit facilities, or any one or more of them, or any part thereof, to any individual, copartnership, or corporation shall be made or entered into, unless said agreement provides that the actual use or operation of such transit facilities, or any one or more of them, or any part thereof, shall begin within a definite fixed and reasonable time. Any of the said agreements may be entered into either prior to the commencement of the said construction work or during the course thereof, or subsequent to the completion thereof.