In this photo provided by the Transport Workers Union, Local 100, workers from the New York Metropolitan Transportation Authority respond to the scene of a subway derailment, Tuesday, June 27, 2017, in New York. A subway train derailed near a station in the Harlem neighborhood of New York, frightening passengers and resulting in minor injuries as hundreds of people were evacuated from trains along the subway line. (Transport Workers Union, Local 100 via AP)

In this photo provided by the Transport Workers Union, Local 100, workers from the New York Metropolitan Transportation Authority respond to the scene of a subway derailment, Tuesday, June 27, 2017, in New York. A subway train derailed near a station in the Harlem neighborhood of New York, frightening passengers and resulting in minor injuries as hundreds of people were evacuated from trains along the subway line. (Transport Workers Union, Local 100 via AP)

NEW YORK (AP) — Investigators probing the derailment of a New York City subway train said the accident was caused by an improperly secured spare rail, left on the tracks to make it easier to fix breakdowns quickly.

Two supervisors responsible for oversight of track maintenance work were suspended without pay pending a formal review, the Metropolitan Transportation Authority announced Wednesday.

MTA Chairman Joe Lhota and Interim Executive Director Ronnie Hakim said in a statement that storing equipment between tracks is a common practice employed by railroads across the country to accelerate repairs.

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“The key to this being an effective and safe practice is making sure that the extra equipment is properly bolted down, which does not appear to have happened in this case,” they said.

The derailment on Tuesday in Harlem caused an A train to list and scrape along a tunnel wall, tossing people to the floor and forcing hundreds of passengers to evacuate through dark tunnels. About three-dozen people were hurt, though not seriously.

Crews were inspecting “every inch of rail” in the system Wednesday to ensure that all replacement parts left near the tracks were properly stored and secured, MTA officials said.

David B. Clarke, director of the Center for Transportation Research at the University of Tennessee, said it was unlikely vibrations from passing trains could have moved such a rail, since it weighs approximately 100 pounds per yard.

“You have to make sure employees are aware that whatever is near the track is not a hazard. In this case, somebody apparently failed,” he said.

The derailment followed a winter and spring marked by mechanical failures, power outages and several episodes in which passengers were trapped on stuck trains for an hour or more.

The subway system, which dates to 1904, operates 8,000 trains daily over 655 miles of track; there are more than 1,600 mainline switches and 13,000 signals that control train movements.

“The pressures and problems of maintaining an older system while still operating 24/7 can lead to something like this,” said Jim Gannon, spokesman for Transit Workers Union Local 100, which represents subway workers but not the suspended supervisors. “Every night, hundreds of them are out doing needed maintenance. They’re under a lot of pressure.”

Service on the four affected subway lines was restored Wednesday morning, though riders were told to expect residual delays.

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An MTA track worker died last year after he was struck by a subway train in Brooklyn while working on a track project.

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Eltman reported from Mineola, New York.

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This story has been corrected to show that investigators said Tuesday the derailment was caused by an improperly secured spare rail, not Wednesday.