But police and paramedics who arrived to try to help the seriously injured teenager as he lay stuck under the train in Taree were pelted with rocks and verbally abused in the early hours of Sunday morning, police say.

Police officers and the train driver initially had to take refuge in the driver's cabin before police reinforcements arrived to protect them from the crowd, which had gathered near the railway crossing on Bushland Drive about 4.30am on Sunday.

 

An ambulance windscreen was also smashed and paramedics were "blocked, pushed and verbally abused" during their initial attempts to treat the teenager, police said.

A NSW Police spokesman said the teenager had been celebrating his 18th birthday with family and friends at a home near the railway crossing on the night of the accident.

At 4.30am on Sunday, police were called to the crossing after reports that a man had been hit by a freight train and ended up under a carriage.

Police arrived to find the 18-year-old lying under the train with both of his legs severed.

The train driver, a 33-year-old man from Queensland, managed to stop the train after the accident and called triple-0.

Police set up a crime scene at the railway crossing.

The police spokesman said officers would investigate the circumstances of the accident, including why emergency service personnel were hindered in their attempts to help the injured man, who was drifting in and out of consciousness at the time.

"Police inquiries will also examine the actions of a group of people who threw rocks at police officers, and the train driver, forcing them to take refuge in the train driver's cabin," police said in a statement.

"Ambulance paramedics were also blocked, pushed and verbally abused by the group during their initial attempts to treat the teen, and an ambulance vehicle was damaged after being hit by rocks."

The injured man was taken to John Hunter Hospital in a serious condition.  It was not clear why he was on the tracks at the time of the accident, the spokesman said.

No emergency service personnel were injured.

The train driver was taken to Manning Base Hospital for mandatory blood and urine testing.