(a)a period before the expiration of which the tramway shall not be commenced, and the conditions subject to which the local authority, when it is not itself the promoter, may, within that period elect to be substituted in the place of the promoter in respect of the undertaking or of so much thereof as is within its circle; and the limits of time within which, and the terms upon which, the local authority may, after the tramway has been constructed, require the promoter to sell to it the undertaking or so much thereof as is within its circle;