The board of equalization shall divide the valuation of each continuous line found and determined by it by the number of miles [kilometers] of such line in the state, and the result is the valuation per mile [1.61 kilometers] for which said line must be assessed. The value of each branch line must be determined in the same manner. Such valuation per mile [1.61 kilometers] must be apportioned to each county according to the number of miles [kilometers] of such line or branch line in the county.