All structures, machinery, dynamos, apparatus, poles, wires, fixtures of all kinds and descriptions, and pipe lines employed in the generation, production, supply, distribution, transmission, or transportation of electric power or natural gas, crude petroleum and refined petroleum products or combinations thereof, shall be taxed as real estate in the town in which said property or any part of it is situated; provided that no electric power fixtures which would otherwise be taxed under this section shall be taxed under this section if they are employed solely as an emergency source of electric power.