CHICAGO (CBS) — The train that hit the officers was a South Shore electric train and it’s quieter than most.

What role did that play in the deaths of the CPD officers in this tragedy?

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As a South Shore train departs its station, the cars pass by with a high tech hum. It’s a sound that continues inside the passenger cars as well.

Dr. Joseph Schofer is a civil engineering professor at Northwestern University.

“The propulsion system is quiet,” said Schofer.

Electric motors on top of each car are what propel them forward, easily capable of speeds of up to 75 miles per hour. But cameras observed near the site of Monday night’s accident, they do make their presence known.

“Still they are electric motors. They are not silent, but they are not anywhere near as noisy as a diesel,” Schofer said.

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The South Shore Line confirmed that its train was traveling at the speed limit of 65 miles per hour at the time of Monday night’s accident.

The South Shore Line estimates it would take a quarter to a half mile to suddenly apply the brakes and stop.

“You can slam on your brakes, but the train weighs very more than a car. It’s steel wheels on steel rails so you don’t have much traction,” Schofer said. “It will take a fair distance to slow that down.”

Metra runs the rails in this corridor. In emergencies like this, protocol calls for Chicago police to contact the 911 center, which contacts Metra police, and then Metra dispatchers who order a possible train stoppage.

But Metra said that’s a call they never received.

The South Shore Line said it extended its condolences to the families of the officers who were killed. The train line also acknowledged the impact on its own staff, particularly on the train’s operators.

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Counseling services are being made available to those employees.