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Key legislators OK’d one of Gov. Charlie Baker’s marquee transit investment bills Wednesday afternoon, while warning that it was merely a down payment on what’s needed to fix Massachusetts’ transportation system.

House Transportation Committee Chairman William Straus told reporters after a private meeting with House Democrats the bond bill would authorize the $18 billion requested by Baker. However, the committee decided to cut Baker’s request to devote half of future Transportation Climate Initiative money to the MBTA because of the uncertainty underlying the regional cap-and-trade program Baker is working to put together to reduce vehicle emissions.

“TCI doesn’t exist, so there’s no way to allocate something that doesn’t exist,” Straus said. “Obviously the most prudent thing is you cannot rely on it for borrowing or going to Wall Street.”

The bill the committee voted on also omits the governor’s plan to expand the use of grant anticipation notes by $1.25 billion to support a $4 billion bridge preservation and reconstruction program over the next eight years.

Large portions of Baker’s bill pay for projects already set in motion, like the purchase of new fleets of Red and Orange Line cars for the MBTA, South Coast Rail and the Green Line Extension. The bill also includes money to find solutions to local traffic bottlenecks and expand the use of bus rapid transit and $330 million to invest in the state’s regional transportation authorities, which operate buses outside of the MBTA service area.

“No one should think this $18 billion solves the problems,” Straus said. “It’s helpful, but it is nowhere near the kind of infrastructure and system needs that the state is need of correcting.”

The Mattapoisett Democrat said the committee was able to keep the total authorization in the transportation borrowing bill on par with Baker’s proposal by “redirecting some older existing bond authority.”

Neither House Speaker Robert DeLeo and senior legislators have been meeting with their colleagues in small groups to hash out a larger plan to fund major investments in the state’s transportation system.

Straus said that he and Ways and Means Chairman Aaron Michlewitz held the last of their meetings with individual House caucuses last Tuesday when they sat down with members of the South Shore caucus, which pressed, among things, for expanded ferry service to Boston.

He said he, Michlewitz and Revenue Committee Chairman Mark Cusack are now in the process of identifying individual revenue ideas and assigning them potential values before looping in DeLeo’s office and others in leadership to assemble a final package.

“I think it’s fair that members would want to see how their districts are brought into the use of the funds,” Straus said when asked about the politics of putting together a bill that can win broad support in the House.

Baker has threatened to veto a gas tax hike, which would require two-thirds in both branches to overcome.