Where a district boundary is shown following a street, highway, road, right-of-way, parkway, public utility right-of-way, railroad, stream or watercourse, the boundary is respectively the center line of such street, highway, road, right-of-way, parkway, public utility right-of-way, main channel of a stream or watercourse or a line located midway between the main track of said railroad, and such boundary shall be deemed to be changed automatically whenever the said center line or the main railroad tracks are changed by natural or artificial means.