All such cities may operate or may lease such subway or subways, or parts thereof, on terms to be fixed by said cities. Such cities may contract for the purchase or construction of such subway or subways, and for lease of same, and may grant rights therein, or in any part thereof, upon such terms as they may deem best, for cars, other means of transportation, pipes, wires and cables used for public service purposes. But no such lease or grant shall be made for a longer period than fifty years. And such cities may regulate the use of such subway or subways, and of the construction and operation of cars, other means of transportation, pipes, wires and cables used therein; provided, that such city shall not lease, grant or let such street railroad, street railroad system, subway or subways, or any part thereof, or rights therein, for any purpose whatever, without the assent of a majority of the qualified voters of such city voting for or against such lease, grant or letting at an election held for that purpose.