In the case of expenditures in connection with the rehabilitation of a unit of railroad rolling stock (except a locomotive) used by a domestic common carrier by railroad which would, but for this subsection, be properly chargeable to capital account, such expenditures, if during any 12-month period they do not exceed an amount equal to 20 percent of the basis of such unit in the hands of the taxpayer, shall, at the election of the taxpayer, be treated (notwithstanding subsection (a)) as deductible repairs under section
162 or
212. An election under this subsection shall be made for any taxable year at such time and in such manner as the Secretary prescribes by regulations. An election may not be made under this subsection for any taxable year to which an election under subsection (e) applies to railroad rolling stock (other than locomotives).