The department shall exercise constant diligence to ascertain the rates, charges, rules, and practices of common carriers operating in this state, in relation to the transportation of freight in interstate business. When it shall ascertain from any source or have reasonable grounds to believe that the rates charged on such interstate business or the rules or practices in relation thereto discriminate unjustly against any of the citizens, industries, interests, or localities of the state, or place any of them at an unreasonable disadvantage as compared with those of other states, or are in violation of the laws of the United States regulating commerce, or in conflict with the rulings, orders, or regulations of the surface transportation board, the department shall take the necessary steps to prevent the continuance of such rates, rules, or practices.