CHICAGO (AP) — Federal safety officials investigating a Chicago commuter train crash that injured nearly 40 people when it slammed into snow-removal equipment are focusing on a “design problem” with its braking system.

National Transportation Safety Board Chairperson Jennifer Homendy said the Chicago Transit Authority train was traveling at 26.9 mph (43.3 kilometers per mile) on Thursday when it struck the snow-removal equipment, which was on the tracks conducting training for the winter season.

She said that based on preliminary information she believes that equipment, with six CTA workers onboard, was stopped when the train crashed into it.

Homendy said NTSB's initial calculations based on the train's speed and other factors such as the number of passengers on board indicate it was designed to stop within 1,780 feet (542.5 meters) to avoid something its path. But that didn't happen, and it crashed into the snow-removal equipment.

“Our team was able to determine that it was in fact a design problem. The braking distance should have been longer,” she said Saturday during a briefing with reporters, adding that a “brand new” system on the same tracks would have had 2,745 feet (837 meters) to stop to avoid a crash.

Homendy said NTSB investigators are “very focused on the design issue and the braking and why the train didn’t stop.” She said they are also reviewing CTA’s braking algorithm to determine whether or not it is sufficient.

Investigators know the train's wheels were slipping as the conductor was braking the train prior to the impact and they have found thick, black “debris residue” on the tracks that are still being assessed, she said.

Homendy said the NTSB has determined there was nothing wrong with CTA’s signal system and how it communicated with the train, but again cautioned that is a preliminary finding that could change.

CTA data shows that during November there have been 50 other times when its trains have had to slow down due to other equipment stopped on the tracks ahead, and none of those resulted in a crash, Homendy said.

She said investigators cannot say yet whether other CTA trains might also have similar braking system issues, but she stressed that CTA’s system is safe.

“I would take the train tonight, tomorrow. I have no safety concerns about taking the train,” Homendy said, noting that 43,000 Americans die in motor vehicle crashes each year.

Homendy said Friday that the NSTB will likely need a year to 18 months to produce a final report with an analysis of what happened, conclusions and recommendations.

In Thursday's crash, the CTA train was heading south from Skokie when it rear-ended the snow-removal equipment on Chicago's North Side. Thirty-eight people were hurt; 23 were taken to area hospitals. No one suffered life-threatening injuries, officials said.