Each corporation operating a railroad, and carrying on business for profit in this state, shall, on or before July first, annually, pay a tax computed upon its gross earnings from all sources from operations in this state; gross earnings being all receipts classified as railway operating revenues by the Interstate Commerce Commission or its successor agency in the classification of accounts prescribed by said commission. No deduction shall be made from such gross earnings for any commission, rebate or other payment, except a refund resulting from an error or overcharge.