Transit officials in New York City on Monday approved a $1.97 billion construction contract to start the majority of the next phase of the expansion of the Second Avenue subway line, a long-promised transportation project that is among the world’s most expensive.

Board members of the Metropolitan Transportation Authority, the state agency that runs the city’s transit system, voted to greenlight the contract, which was awarded to Connect Plus Partners, a partnership between two construction firms. Early work is scheduled to begin later this year, with the digging of a new tunnel set to start in 2027. This latest agreement is one of the four construction contracts that will make up the second phase of the project, according to an M.T.A. presentation made to transit officials last week.

The first phase of the subway line opened in 2017 and stopped at East 96th Street on the Upper East Side of Manhattan. Politicians have promised for nearly a century to extend the line farther north to East Harlem, a working-class enclave. The planned extension will add three subway stations along the Q line at 106th, 116th, and 125th Streets. It will give a historically neglected community more transit options and shift passengers away from some of the country’s most crowded train lines.

But the project was thrown in doubt last year. The M.T.A. paused work on the roughly $7 billion project in June 2024, after Gov. Kathy Hochul indefinitely suspended the congestion pricing tolling program, which was expected to help provide roughly $3.5 billion for the subway line extension. The tolling program, which charges drivers to enter Manhattan below 60th Street, eventually began in January after Ms. Hochul reversed course.

Ms. Hochul, a Democrat, announced the approval of the contract on Monday after the vote.

“It has been a century since the people of East Harlem were promised the new subway they deserve, and we are finally getting it done,” she said in a statement.

The latest contract funds the digging of tunnels between the planned stations at 116th Street and 125th Street, as well as a connection between the subway line and the Metro-North station at 125th and Park Avenue, the governor’s office wrote in a news release. The station at 106th Street will be built under a different contract.