(a) “Gross revenue” shall mean and include all earnings or revenue derived from the use or operation of freight cars, as hereinafter defined, upon or over the lines of any railroad company, as hereinafter defined.

(b) “Gross revenue in this State” shall mean and include (a) all gross revenue on intrastate business and (b) a portion of the gross revenue on all interstate business passing through or into or out of the State, based, in each instance, on the proportion of mileage over which such business is done within this State.

(c) “Freight cars” shall mean and include all stock cars, furniture cars, refrigerator cars, tank cars, or any other kind of cars used to transport any commodity over the lines of any railroad company in this State, as hereinafter defined. All such freight cars are hereby declared to have, and are hereby given a situs for taxation purposes in this State. This Act does not include (1) cars owned by an express company, or (2) cars owned by a sleeping-car company, such as the Pullman Company, or (3) cars owned by a railroad company.

(d) “Company” shall mean and include all persons, firms, associations and corporations.

(e) “Freight line company” shall mean and include all companies engaged in the business of operating freight cars or engaged in the business of furnishing, renting or leasing freight cars for the transportation of freight (whether such cars be owned by such company or by any other person or company) over any line of railroad, in whole or in part, within this State, such line or lines not being owned, rented, leased or operated by such company.

(f) “Equipment company” shall mean and include every company engaged in the business of furnishing, renting or leasing freight cars to be used in the operation of any line of railroad wholly or partially within this State, such line or lines not being owned, leased or operated by such company.

(g) “Mercantile company” shall mean and include every company whose principal business is other than that of a freight line company or equipment company, as hereinbefore defined, but which owns, operates, leases, rents, or otherwise uses any freight cars in the operation of its business.

(h) “Railroad company” shall mean and include every steam railroad, street railway, or interurban railway company operating or doing business in this State as a common carrier.