TTC CEO Rick Leary apologized Thursday after a leak Monday shut down subway service on a busy stretch of Line 2 (Bloor-Danforth) for half the day and said he's ordered a forensic investigation into what went wrong.

"Accountability ultimately falls with me as the CEO and I do understand that," Leary told reporters following a TTC Board meeting. "And I think that you saw and heard from our staff today. They're taking accountable. This had a really negative impact on service and for that I greatly apologize."

Service on the busy subway line was disrupted for 12 hours Monday between St George and Broadview stations after a hydraulic oil leak from a work car caused slippery tracks.

Crowds of people were forced to wait on the surface for shuttle buses as the problem dragged on through the morning and afternoon rush periods.

The incident followed other recent problems on the busy east-west transit line, including a recent fire, which shut down service for days on a portion of the tracks in the city's west end.

Leary said Thursday that some 200 litres of the fluid leaked in the incident Monday, leaving TTC employees to manually scrub the rails as they scurried to try to restore service.

He said he first became aware of a problem with such leaks in April and said the system has seen seven of them so far this year. He couldn't immediately say whether all of the leaks have been on Line 2.

"I've never seen that, what occurred on Monday before; 12 hours out of service," Leary said. "We had staff out scrubbing the rails by hand. That's unheard of."

He said he's ordered the forensic investigation by experts to find "the root cause" of the problem.

Speaking with reporters, TTC Board members said the incident raises concerning questions.

"There are a lot of questions that need to be asked," TTC Chair Jamaal Myers said. "And there's a lot of concern over this incident; how it happened and that this is not the first time we've had a hydraulic oil spill this year."

Mayor Olivia Chow said earlier this week that every day members of her staff "cross their fingers to hope that they can get here on time and not experience another shutdown."

She apologized for the shutdowns and acknowledged that getting to work on time Monday "was close to impossible" for those relying on Line 2.

"Keep taking the TTC. It is still the better way. We apologize for the shutdowns," Chow said ahead of a meeting of her executive committee Tuesday. "We literally need more investment. And without that, it's not as reliable as we want. But still it's a very good system."

While Chow said the problem is lack of investments, Coun. Josh Matlow, who sits on the board, said he's concerned that some of the information about the leaks is just now coming to light.

"If there was just one isolated incident, I think more people can understand that it's just something that needs to get fixed," Matlow said. "What is coming out is that there have been repeated incidents of hydraulic oil leaks over the past few months, and it just keeps happening along with other maintenance issues, which begs the question, is there something bigger that's going on that has not been disclosed to the public, to the TTC Board? What is management doing to actually own the problems and fix it?"

According to the latest figures published by the TTC, Line 2 saw more than 40 hours worth of delays in March, a 26 per cent increase from the previous month. However the line's on-time performance was still 90.4 per cent that month, just over the 90 per cent target set by the TTC.