United States Code, shall remain available to the Secretary for obligation under that section for a period of 4 years after the last day of the fiscal year for which the funds are authorized. (2) Transfer of unobligated funds Any amounts made available under subsection (a) to carry out section 5309 of title 49, United States Code, that remain unobligated at the end of the 4-year period referred to in paragraph (1) shall be deposited in the Highway Trust Fund. 4. Establishment of trust fund (a) In general Subchapter A of chapter 98 of the Internal Revenue Code of 1986 is amended by adding at the end thereof the following: 9512. National infrastructure investment trust fund (a) Creation of trust fund There is established in the Treasury of the United States a trust fund to be known as the National Infrastructure Investment Trust Fund , consisting of such amounts as may be appropriated or credited to the National Infrastructure Investment Trust Fund as provided in this section or section 9602(b). (b) Transfer to national infrastructure investment trust fund of amounts equivalent to certain taxes There are hereby appropriated to the National Infrastructure Investment Trust Fund amounts equivalent to 25 percent of the portion of the taxes received in the Treasury under the rates described in clauses (i) and (iii) of section 4081(a)(2)(A), and section 4081(a)(2)(D), but only to the extent such amounts are attributable to the increase in rates under such clauses, and such section, by reason of the amendments made by section 5 of the Build America Act of 2019. For purposes of the preceding sentence, taxes received under section 4041 and 4081 shall be determined without reduction for credits under section 6426. (c) Expenditures From National Infrastructure Investment Trust Fund Amounts in the National Infrastructure Investment Trust Fund shall be available, without further appropriation, to the Secretary of Transportation for making expenditures after October 1, 2020, under section 5309 of title 49, United States Code,