((a)) The Board shall, as often as practicable, but in no event less often than quarterly, publish a rail cost adjustment factor which shall be a fraction, the numerator of which is the latest published Index of Railroad Costs (which index shall be compiled or verified by the Board, with appropriate adjustments to reflect the change in composition of railroad costs, including the quality and mix of material and labor) and the denominator of which is the same index for the fourth quarter of every fifth year, beginning with the fourth quarter of 1992.
((b)) The rail cost adjustment factor published by the Board under subsection (a) of this section shall take into account changes in railroad productivity. The Board shall also publish a similar index that does not take into account changes in railroad productivity.