7 For the purposes of sections 264, 272, and 276 to 278 or for other purposes pertaining to rates for the carriage of goods,
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(a) costs shall be variable costs based either on the expense accounts maintained under the Uniform Classification of Accounts and accounts reconcilable therewith or on such special studies of items and factors of costs as the Committee orders pursuant to section 46 of the National Transportation Act, and shall include the increases or decreases in rail operations expenses resulting from changes in the volume of traffic, after allowing a reasonable period of time for adjustment in view of the traffic to be handled;