As congestion pricing makes a comeback, there is a lot at stake for New York City’s aging mass transit system.

Subways that need modern signals to run on time. Stations that lack elevators. A long-awaited expansion of the Second Avenue subway line into Harlem.

The Metropolitan Transportation Authority is counting on the tolling program to raise $15 billion to fund a host of crucial repairs and upgrades. But a revised plan announced last week by Gov. Kathy Hochul would slash the tolls across the board by 40 percent, with most drivers paying $9 to enter the busiest section of Manhattan at peak periods, down from the original charge of $15.

Would that still bring in enough to help maintain and modernize the transit system? Governor Hochul and transit officials say yes, but some who have been monitoring the tolling program’s evolution — both supporters and opponents — are not so sure.

“I think they may well be able to fully plug the $15 billion hole,” said Andrew Rein, president of the Citizens Budget Commission, a watchdog group that supports the revised program. “But until we see the details, we won’t know if there’s any risk to the timing of critical projects and whether it would cost any extra money.”

Ms. Hochul resurrected congestion pricing last week after abruptly canceling it in June just weeks before its start date. By reducing the tolls, she attempted to reach a compromise between the transit authority’s vast needs and the many critics who complained that the plan’s original tolls were too high. The agency’s board will vote Monday on the new plan, which is expected to start in January.