The selfie was shared just moments before their tragic deaths

Utah County sheriff's Sgt Spencer Cannon said that the girls were waving to the engineers of the Utah Railway train as it passed by.

The trio had been taking photos, and knew that one train was approaching, but did not realise that there was another one coming in the opposite direction.

When the Union Pacific train came from the other direction, there was only a three or four foot gap between the two trains, with Cannon saying the force was strong enough to buffet the three girls.

Cannon said that the engineer on the eastbound train sounded the horn as he saw them between the tracks, but they failed to respond.

"He told deputies the girls did not respond to his blasting of the horn and he was unable to stop before hitting them," he said.

The train conductor who was working at the time, John Anderson, said the girls were in "their own little world".

"We watched in horror as we got closer," John said. "We saw them for about 12 seconds until they disappeared from our sight and the train continued moving forward."

After the train came to a halt, he ran over to the girls to help. He realised two of the girls had no pulse, but Savannah had survived.

I told her everything would be OK and she relaxed a little," he said. "I hoped she would make it and for some reason I really thought she would."

Savannah was taken to hospital and had surgery, but the 13-year-old was taken off life support after doctors said she would not recover from severe brain injuries.

"We spoke to the doctors today and they informed us that Savvy's brain injuries were too great," her mum Jayna Webster wrote at the time.

"Even though they've had done everything possible, Savannah will not be able to recover any further. We will keep her on life support till the end of the day, but it's time to say goodbye, for now, to an angel that walked among us."

Just moments before the girls' tragic deaths, Savannah shared a selfie to Facebook, accompanied by the words: "Standing right by a train ahaha this is awesome!!!!"

The image shows the train approaching behind them, with the lights glowing above their heads.