The 2[Government] may, with the consent of the local authority and road-authority and of the promoter and his lessee (if any), extend any part of this Act, or any rules made under this Act, either with or without modification, to the whole or any part of a tramway constructed, or authorised by the 2[Government] to be constructed, before the passing of this Act, and may withdraw any part of the Act or any rules so extended.