CHICAGO (AP)  -- The track at the site of a subway derailment that sent more than 150 people to the hospital was too wide prior to the accident, a National Transportation Safety Board investigator said Friday.

The track exceeded the standard rail-to-rail width at several points by up to 1 1/8 inches, NTSB investigator Kitty Higgins said.

``That doesn't seem like a lot, but that's enough for the car to derail,'' she said.

Investigators have not reached their final conclusions on the accident's cause, Higgins said.

Chicago Transit Authority track is supposed to be examined twice a week, and the section of track where the derailment occurred was inspected July 7 - four days before the evening rush-hour accident, Higgins said.

Investigators now will try to determine why the tracks were too wide, she said.

The last car of the eight-car train heading to O'Hare International Airport went off the track Tuesday, causing material underneath to catch fire. As many as 1,000 people may have been on the train; more than 150 were sent to area hospitals.

Drug and alcohol tests on the train's operator taken after the accident came back negative, Higgins said. The NTSB also has said the operator told investigators he believed he was going 20 mph at the time of the accident - 5 mph below the speed limit.

Most of the passengers treated at hospitals following the Chicago subway train derailment suffered from smoke inhalation. It's a misunderstood condition that causes more deaths after fires than burns.

Experts say smoke plays a deadly game with people escaping from fire. It distorts their perception of time, entices them with drowsiness and robs them of oxygen.

Doctor Michael Silver of Rush University Medical Center says about five-thousand people die annually in the U-S from smoke inhalation.

Silver is a lung specialist. He says Tuesday's quick evacuation from the train and tunnel, followed by immediate medical care probably saved most passengers from long-term health consequences.

The one passenger who remains hospitalized in critical condition today is an elderly woman who is not a smoke inhalation patient.