All freight cars owned, operated, rented, leased, or used by any freight line company, equipment company, or mercantile company which are moved over, or used in the operation of, the line of any railroad company, as hereinbefore defined, wholly or partially within this State, are hereby classified for the purpose of taxation; and a tax equivalent to four (4%) per cent of the gross revenue in this State, is hereby levied on such freight cars; and such tax shall be in lieu of ad valorem taxes upon such freight cars.

Nothing in this act shall be construed to exempt from ad valorem taxation any real or personal property other than freight cars, or any freight cars which are not operated over the line of any common carrier railroad, as hereinbefore defined, upon which the gross revenue tax herein levied does not apply. It is hereby expressly provided that the provisions of this Act shall apply to both public service and private corporations.