Metro said Thursday it may have figured out what’s causing wheelsets to fall out of spec on its problematic 7000-series railcars.

The cars, which make up 60% of Metro’s rail fleet, were pulled from service following a Blue Line train derailment in October.

Metro General Manager and CEO Paul Wiedefeld told the agency’s board of directors that “probable root causes” of the wheelset issues had been identified.

“It is very likely that a combination of issues contributed,” he added, without going into further detail.

Wiedefeld said the next step will involve testing to validate or reject their theories.

The news was greeted with enthusiasm by Board Member Tracy Hadden Loh.

“Obviously, it’s absolutely massive to make progress on finding the root cause,” she said.

Wiedefeld also said future updates could include information about a timeline for returning the problem railcars to service.