WASHINGTON — The Trump administration is halting reimbursements on nearly $18 billion in federal commitments to two of New York City’s largest infrastructure projects — the Hudson Tunnel and the Second Avenue Subway expansion — pending a review of contracting practices. The projects are part of the multibillion-dollar Gateway Program and the Metropolitan Transportation Authority’s long-delayed Second Avenue subway, both of which rely heavily on federal support. That includes a $300 million payment for the Second Avenue subway that was scheduled for release this year. The Hudson Tunnel Project is central to Amtrak’s Gateway Program, which would add a new two-track tunnel under the Hudson River and eventually double rail capacity between New Jersey and New York. The Second Avenue Subway project aims to relieve congestion on Manhattan’s Lexington Avenue Line by extending service north through East Harlem, with new stations planned at 106th, 116th, and 125th streets.