The Chicago Transit Authority hopes an investigation will provide answers as to what caused pieces of steel to come loose from the track Tuesday night causing a train derailment in the Loop.
No one was injured after two of the eight train wheels derailed on the northbound Orange Line elevated tracks near La Salle Street.
But people walking along the street below were forced to take cover as chunks of steel and wood fell from the train platform.
"It was just loud. Kind of a bang, but it continued and it definitely shook the building," said pedestrian Pamela Tolby.
As the train derailed, it apparently pulled out some steel rail fasteners that hold the track in place. CTA officials say this is uncommon.
"No doubt this needs to be looked at when you have multiple objects fly to the street," said DePaul University professor and transportation expert Joseph Schwieterman.
The curved steel rail fasteners are commonly used, but don't typically come loose. Schwieterman says the malfunction could have had something to do with the age of the system.
"This is one of the challenges," Schwieterman said. "We have a really old system. We have an el dating back to the 1890s. We are still using wooden cross ties. All the rails still have wooden platforms."
Schwieterman also said there's less data about elevated train tracks because they are not as common.
"What probably happened is the wheel ran along these wooden cross ties and just tore up the wood, wood splinters flying everywhere, and then these metal fasteners came down with that," Schwieterman said.
CTA official Brian Steele said there was no structural damage to the track itself. All service in the Loop was fully restored by 3:30 a.m. Wednesday.