The unprecedented Orange Line shutdown is more than halfway completed and officials say work is on the right track. But, one transportation expert who warned of MBTA disaster 13 years ago said a true fix for the T will take decades and billions of dollars.

David D’Alessandro, the lead author behind a 2008 MBTA review under then-Governor Deval Patrick, said on Greater Boston that Gov. Charlie Baker has done a good job of trying to improve the MBTA, but "it ain't enough."

D’Alessandro said it will take billions of dollars and decades of work to fix the public transportation system. "We're in for a very long ride, maybe 20 years," he said.

After D'Alessandro published his 2008 MBTA report, he said not a single legislator reached out to him. When asked by Jim Braude if the MBTA would struggle with issues for the next decade, D'Alessandro said yes.

D'Alessandro said Massachusetts legislators complain about the MBTA frequently, but have underfunded it for years and have not been held accountable by voters.

WATCH: Massachusetts Governor Charlie Baker says MBTA is on the right track. But is it?