DENVER (AP) — Police and federal railroad officials are investigating how a motorist ended up driving through a railroad crossing Tuesday and colliding with Denver’s airport train. The van’s driver was killed in the collision and four train passengers suffered minor injuries.

The train has had problems with its automated crossing gates since it began running nearly a year ago, requiring its operator to post off-duty police officers and flaggers at crossings to keep drivers off the tracks as a backup safety measure.

An officer and a flagger were working at the crossing in suburban Aurora when the van drove toward it as a train bound for Denver International Airport approached at about 3:50 a.m., police Sgt. Chris Amsler said. The officer tried to stop the van but it kept going and was hit by the train, which pushed the van for about a quarter of a mile before the conductor was able to stop it.

Three of the injured were treated at the scene but one person about the 56-passenger train was taken to the hospital.

The crossing gates were working but it was not clear when they started to go down. They were down when the train hit the van, Amsler said.

The Denver area’s Regional Transportation District opened the airport train line in April 2016 despite problems during testing with crossing gates going down when trains were not passing or not fully going down when they were. State regulators required posting workers at the crossings as a temporary solution.

Because of the crossing problems, the Federal Railroad Administration that regulates the 23-mile line has granted several 90-day operational waivers to allow the airport trains to run while RTD and its contractor, Denver Transit Partners, work to resolve the problem. The most recent waiver was granted in January.

The airport trains have average weekday ridership of about 20,000 passengers, RTD has said.

There have been other problems since line opened, including sporadic delays and power outages.

In May, about 80 passengers were evacuated when a train became stranded on a 50-foot-high bridge.

The cause was lightning that severed a wire that supported electricity lines, disrupting power used by the electric powered trains, RTD said.

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