All property of railroad companies shall be assessed, and the taxes collected in the several counties in which said property is situated, including so much of the roadbed and fixtures as shall be in each county. The rolling stock may be assessed in gross in the county where the principal office of the company is located, and the county tax paid upon it shall be apportioned as provided by general law in proportion to the distance such road may run through any such county, among the several counties through which the road passes, as a part of their tax assets.