THE TRIPCOCK DID ITS JOB. The preliminary investigation of the CTA derailment under Lake Street reveals that the train derailed in such a way that the brake tripper activated and stopped the train.

Investigators had not yet determined what caused the fire, but they said they think a train trip switch, located at the back of each railcar, likely opened during the accident, said Kitty Higgins, a member of the NTSB. The switch is designed to work in conjunction with the signal system and activates the brakes if, for example, a train operator doesn't heed a red light, they said."We believe the derailment essentially tripped the train trip switch. When the car went off the tracks, it hit the ground, the ties, the concrete, something with enough force that it opened this up," Higgins said, holding the small device.When the trip switch opened, the electrical current was interrupted and an emergency brake system was automatically triggered, she said.

Higgins said the trip switch is not designed to open in a derailment, and the NTSB's investigator-in-charge, Mike Flanigon, said he's never seen a trip switch open in a derailment in his 20-plus years of experience.

Rapid transit equipment often has a trip switch designed to operate in conjunction with a track trip sometimes referred to as a "rooster" that is interlocked with an absolute stop signal. It rises when the signal is at stop, to activate the trip switch and stop the train. (Mainline railroads cannot use this device as the mass of a heavy road locomotive can overwhelm the rooster, and an extremely sensitive trip switch -- the obvious response -- is prone to stopping the train at the least provocation. Rapid transit trippers and roosters can be optimized to the weight of representative trains more easily. That does not preclude false deployment. A story has been passed along from the North Shore Line, which was required to cut in the trippers on the Rapid Transit. A large rabbit that was running between the tracks ahead of the train ran afoul of the trip switch, which interpreted the rabbit as a rooster and activated the brakes.)