The state tax commission shall apportion the aggregate value of all distributable property of such private car companies, except such valuations as are found to be de minimis, to each county, showing the valuation apportioned for taxation by each political subdivision in the county authorized to levy ad valorem property taxes. Such apportionment shall be determined by the proportion of main line miles of track in each county to the total mileage of main line track in the state of each of the railroad or street railway companies which have transported railroad cars in behalf of such private car company in the prior year. Valuation apportioned for taxation in behalf of school districts shall be the same as that apportioned for each county, and such valuation shall be taxed using an average school tax rate in each county in the manner provided in chapter 151 with the resulting revenues distributed as provided in chapter 151.