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 Funds (a) In general Subchapter A of chapter 28 of the Internal Revenue Code of 1986 (relating to the trust fund code) 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 the portion of the taxes received in the Treasury under subsections (a)(1) and (b) of section 5701 that are attributable to the increase in tax imposed by such subsections by reason of the amendments made by section 5 of the Build America Act of 2018. (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 under the national infrastructure investment program authorized by section 2 of the Build America Act of 2018. 9513. Fixed Guideway Capital 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 Fixed Guideway Capital Investment Trust Fund , consisting of such amounts as may be