Miss. Code Ann. § 27-35-505

Every company engaged in the business of operating cars, not otherwise listed for taxation or taxed in Mississippi, for the transportation of freight, whether such freight be owned by such company, or any other person or company, over any railway line or lines, in whole or in part within this state, such line or lines not being owned, leased, or operated by such company, whether such cars be termed box, flat, coal, ore, tank, stock, gondola, furniture or refrigerator cars, or by some other name, shall be deemed to be a freight line company.