(CBS) — A CTA Blue Line train gave riders a scare Wednesday, as it thundered toward the Loop with some of its doors open.

At least one set of doors remained open when the train left the Damen station inbound around 8:30 a.m.

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Rider Elizabeth Kenefick tagged her tweet “#mynightmare.” Yet another rider, Eve Adams, wrote,”This train started rolling from Damen at full speed with its doors WIDE OPEN. Go home @cta. You are drunk.”

Perhaps the most dramatic tweet was posted by rider Jack Silverstein. He snapped a photo of the open doorway, with an apartment building whizzing by, and tweeted, “Blue Line just flew halfway between Damen and Division with doors open. Mass panic ensued.”

CTA said only the one set of doors was open on the crowded eight-car rush-hour train, bound for Forest Park, which ran express from Damen to the Loop and was then taken out of service at Monroe/Dearborn.

“Safety is our number one priority, and we are continuing to investigate what occurred,” spokesman Jeff Tolman said.

He said first indications are that a mechanical issue “may have prevented proper operation of the doors.”

The doors and motor controls aboard the 2600-series ‘L’ cars are supposed to be interlocked. If doors are opened, the train is not supposed to move if stopped, or go into emergency braking mode if running.

Riders indicated that a first attempt to pull an emergency braking knob on the car was unsuccessful. A second attempt halted the train, and the operator informed the CTA control center about the problem. A CTA employee entered the car and stayed with it until it was taken out of service.

The 2600 series ‘L’ cars are the oldest still in regular passenger service on the CTA. They were built between 1981 and 1987, some by the Budd Co. and the rest by successor Transit America, both of which are now defunct.

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Tolman said such incidents are “extremely rare,” but it happened aboard a train that WBBM reporter Steve Miller rode two years ago on the Brown Line.