The train, carrying 30 passengers and an operator, collided with a snow removal machine near the Howard Station on Nov. 16. Sixteen people were hurt, three of them critically.
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The NTSB said the operator knew the snow removal machine was on the tracks for a training exercise, but wasn't sure where.
The operator received a stop command from the CTA's signal system because of the snow removal machine being more than 2,000 feet ahead.
Moments later, the driver saw the machine and initiated the emergency brakes, but the train still crashed into the machine, going 27 miles-an-hour.
Its speed had decreased from 55 mph.
The Yellow Line is still out of service, but shuttle bus service remains available.
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The head of the NTSB held a news conference Tuesday afternoon on the report.
The NTSB said it is up to the CTA to determine when the Yellow Line can run again.
NTSB chair Jennifer Homendy said the CTA sent them a memo outlining safety precautions the agency has taken.
"They have reduced the maximum allowable speed on the Yellow Line from 55 mph to 35 mph, and reduced the speed at this particular area of the accident to 25," Homendy said.
On Tuesday afternoon, the CTA released a new statement, saying, in part "The CTA is currently engaged in an extremely thorough review of all aspects of the Yellow Line mentioned in the NTSB preliminary report, from signals to tracks to equipment, as well as testing trains to ensure safe operation. These activities require time to perform, and once this review is complete, CTA will determine a plan to reopen."
NTSB officials plan to return to the Yellow Line next week to conduct more tests.
"We will wet the track to see what sort of response we get during braking to see if we can repeat some of the wheel slippage that we had seen during braking," Homendy said.
NTSB officials said they don't know if CTA's emergency braking equipment on all trains system-wide has been updated over the years to prevent this kind of crash.
The Yellow Line train that crashed had what's called Automatic Train Control, but the signal equipment hadn't been updated, according to federal authorities. They say the train couldn't be stopped in time.
Nearly four weeks later, the chair of the NTSB made a blunt assessment at a news conference announcing findings from the agency's preliminary report on the accident.
"We are not saying the line is safe," Homendy said.
As the I-Team first reported on the day of the crash, the NTSB recommended in 2009 that transit systems, including CTA, install a more modern emergency braking system, called Positive Train Control.
Then in 2014, following a horrific crash at the O'Hare station, CTA officials were told again by NTSB that it needed Positive Train Control.
CTA didn't do it, and still hasn't.
In a 2022 memo obtained by the I-Team, CTA's chief safety officer said to the NTSB that the recommended safety equipment "would require most of the CTA's current capital budget."
Once again, Positive Train Control was not installed.
Federal authorities say if CTA had the modernized equipment on the Yellow Line train, it would have prevented the November crash.
DePaul University Transportation Professor Joe Schwieterman told the I-Team accidents such as this can move the needle.
"Actions like this really do raise the pressure to adopt Positive Train Control everywhere," Schwieterman said.
Despite NTSB's criticism of CTA emergency braking equipment, federal officials stress that passenger rail travel overall is safe.
CTA officials have said for years that federal regulations don't mandate them to have equipment such as Positive Train Control, even though Metra and Amtrak are required to have it.
The I-Team asked the CTA whether braking equipment system-wide has been updated since last month's crash.
The agency said NTSB requirements block them from speaking about ongoing investigations.
At least four lawsuits have been filed in connection with the crash.
The incident caused nearly $9 million in damage to CTA equipment.
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Read the CTA's full statement below:
"The CTA acknowledges publication of the National Transportation Safety Board's (NTSB) preliminary report, and appreciates their ongoing review of the November 16th Yellow Line incident.
"We continue to work closely with the NTSB on its ongoing investigation of the incident. From the beginning, the CTA has been fully committed to assisting the NTSB investigation in any way possible. We continue to provide the agency full access to equipment, facilities, records, personnel and any other information it requests.
"The CTA is currently engaged in an extremely thorough review of all aspects of the Yellow Line mentioned in the NTSB preliminary report, from signals to tracks to equipment, as well as testing trains to ensure safe operation. These activities require time to perform, and once this review is complete, CTA will determine a plan to reopen. Safety continues to be our No. 1 consideration.
"While testing continues, CTA continues to provide free shuttle buses for Yellow Line customers, connecting the Howard station and the two Skokie stations, Oakton-Skokie and Dempster-Skokie."