CHICAGO (CBS) — A new report says road salt is to blame for the failure of crossing gates and warning lights Monday morning at the site where two cars hit a stopped train.

The accident was caught on surveillance tape, which shows a car driving right into the tanker train that was stopped at the Halsted Street crossing near 90th Street. It was dark and the train cars are painted black, suggesting the drivers couldn’t see them.

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The same happened soon afterward on the other side of the train.

One of the cars, a blue Nissan Maxima, hit the connection between the rail cars in the accident just after 3 a.m. Monday. Another, a beige Buick LeSabre, was wedged under the train.

There were no warning lights flashing, and the crossing gates were up.

Federal railroad experts tell the Chicago Tribune the gates and other warning devices weren’t working because of road salt. The salt had built up and caused the circuits to short.

But area residents say the signals are often a problem. Len Torrence says he sometimes has to hold the gate up physically.

“A lot of times we come out here for the buses go through, we might hold it up for them to let them go to go around,” he said. “We’ve done that for cars to keep traffic from backing up so bad.”

Authorities say when train engineer noticed the gates weren’t working, he threw down flares.

Then, as the train was moving eastbound, it stopped to let another train pass. That is when police say the cars collided into the train.

A fire official said even with the engineer throwing down the flares, the train wasn’t visible enough to motorists.

Four people were injured in the accident, and three of them were taken away in critical condition. The fourth was in serious, but stable, condition. Two were taken to Advocate Christ Medical Center in Oak Lawn and two were taken to Stroger Hospital of Cook County, Fire Media Affairs Chief Joe Roccasalva said.

Updated conditions were not immediately available.

A Level 1 Hazardous Materials response was also called, since tanker cars were involved. The tanker cars were carrying denatured alcohol, so there was no serious threat to neighbors from fumes or toxicity.

Denatured alcohol is commonly used as a sanding aid, solvent or fuel for camp stoves.

The Illinois Commerce Commission is investigating this train collision, among other agencies.

The Sun-Times Media Wire contributed to this report.