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CHICAGO -- An emergency track-side braking system activated but failed to stop a Chicago commuter train from jumping the tracks and barreling to the top of an escalator at O'Hare International Airport, a federal investigator said Tuesday.

The events that led to Monday's accident, which occurred around 3 a.m. and injured more than 30 passengers, might have begun with the train operator dozing off toward the end of her shift, according the union representing transit workers. But Tuesday's announcement that a piece of emergency safety equipment might have failed was the first indication the accident could have been caused by human error and mechanical failure.

National Transportation Safety Board investigator Ted Turpin said a preliminary review showed the train was traveling at the correct speed of 25 mph as it entered the station. Investigators said they have not yet determined whether the operator ever applied the in-cab brake.

Turpin, who is in charge of the investigation, said an automatic emergency braking system located on the tracks was activated but failed to stop the train as it burst onto the platform.

"It activated," Turpin said of the emergency system. "That's all we know factually. Now, whether it did it in time or not, that's an analysis that we have to figure out."

A team from the NTSB was also exploring how rested the train operator was before starting her shift and whether rules governing overtime had been violated, after a union official suggested she might have dozed off.

They planned to interview the train operator Tuesday afternoon.

"We're going to ask probably the operator how they felt ... because we always take into consideration the fatigue factor. It's one of the things we do investigate," Turpin said.

The operator, whom officials have not identified, was off duty for about 17 hours before starting work around 8 p.m. Sunday but had recently put in a lot of overtime, Amalgamated Transit Union Local 308 President Robert Kelly said Monday.

"I know she works a lot - as a lot of our members do," he said. "They gotta earn a living. ... She was extremely tired."

Kelly said the operator took standard drug and alcohol tests after the derailment and that she assured him they were not an issue.

Asked whether she may have nodded off, Kelly responded: "The indication is there. Yes."

Federal investigators hoped to turn the scene over to local officials later Tuesday to begin removing the train from the escalator at the underground Chicago Transit Authority station.

The train is designed to stop if operators become incapacitated and their hand slips off the spring-loaded controls. Kelly speculated that, upon impact, inertia might have thrown the operator against the hand switch, accelerating it onto the escalator.

Transit officials refused to discuss what other safety mechanisms are in place around the transit system while the investigation was ongoing.

Federal safety regulators keep a close watch on longer distance, city-to-city passenger rail and freight operations. But federal safety oversight of transit systems within cities has been weaker, and responsibility for any technology to prevent crashes and control speeds has been left to local authorities.

There are efforts to grant a safety oversight role to the Federal Transit Administration, which has primarily been a funding agency, said Sean Jeans-Gail, vice president of the National Association of Railroad Passengers, a Washington-based advocacy group.

In the meantime, local transit agencies like Chicago's make their own choices about how to spend scarce funding, juggling the needs of safely maintaining systems that are a century old in some places with pressure to expand systems to meet demand.

"It's always going to be a tension, but it's a tension that becomes more pronounced when there's not a healthy level of investment in both maintenance and ... capacity expansion," Jeans-Gail said.

Investigators have also been scrutinizing the train's brakes, track signals and other potential factors while reviewing video footage from more than 40 cameras in the station and on the train, Turpin said.

The station remained closed Tuesday, and CTA buses took passengers to and from O'Hare to the next station on the line. Transport officials have not said when full Blue Line service will resume at O'Hare.

Also Tuesday, attorneys for one of the injured passengers filed a negligence lawsuit in Cook County court against the CTA seeking more than $50,000 in damages. A CTA spokeswoman said the agency doesn't comment on pending litigation.