Two people were injured Thursday, one critically, when a vehicle was apparently rear-ended by another at a Riverhead railroad crossing -- then propelled onto the tracks, where it was slammed by a train, authorities said.

At least one car, a Mercedes that was rear-ended, was hit about 1:45 p.m. by a westbound-train out of Greenport, while a Honda sport utility vehicle also careened over the tracks.

The Mill Road crossing gates were down when a Honda going 15 mph hit the Mercedes, the only car stopped in the northbound lane, said Danny Spring, 31, of Hampton Bays.

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He had just crossed the tracks and was going south past the Mercedes when he looked in his rearview mirror and saw the Honda hit her, he said.

"She was eating ice cream," Spring said. "She was just sitting there . . . I go 'Oh man, that SUV's not slowing down too much.' It hit her in the back and pushed her between the two rails that were down."

He said he and others ran toward the Mercedes, pushed west of the gates by the accident, and the Honda, whose driver had apparently lost control and gone over the tracks.

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But they could hear the train's horn "blaring."

Ten seconds later, the Mercedes was slammed a second time with the driver inside, Spring said.

"The car just looked like it got engulfed by the train, like it was swallowed," he said.

One driver was airlifted to Stony Brook University Hospital and was in critical condition, while the other victim was in good condition, a hospital spokeswoman said.

LIRR officials said five people were in the two vehicles.

About 20 people were on the train but none was hurt in the collision, Riverhead police Sgt. John Devereaux said.

Passengers were not allowed off the doubledecker train right after the accident but some could be seen on their cell phones, pacing in the aisles and looking outside.

LIRR spokeswoman Marjorie Anders said it was unclear whether the train hit both vehicles. She said MTA police would not release further details until evidence, such as paint chips off the train and the engineer's statement, had been analyzed.

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Spring said he was focused on the Mercedes and could not see whether the Honda was also struck.

But the SUV's air bags deployed, and they hid the fact that the driver wasn't the only occupant, Spring said. An assistant conductor pushed the air bag off the driver, he said, while another witness, a young woman, spoke to 911.

Spring said he headed home and learned from the news that both victims were alive: "I just hope they stay alive."

With Will James and

Alfonso A. Castillo