The all stations from Leppington to Richmond service took left its penultimate stop at East Richmond on Monday morning with its driver looking towards the terminal and its passengers oblivious to what was about to occur.

Within moments, passengers "were just flying through the air" as the train crashed into the concrete buffer stop on the station's platform 2, stopping it from crashing through the end of the line and onto a busy street.

"We came to this sudden stop, it felt like the whole train had lifted up in the air," passenger Jaiden Ruttley said.

"I was sitting down and was lucky enough to grab a pole before I hit my head on the wall.

"All you could hear was the screaming and people in pain."

NSW Ambulance incident commander Superintendent Paul Turner said 15 people had been transported to four hospitals, the most serious a 21-year-old man who suffered a leg injury.

The other patients suffered back and neck pain, cuts and bruising after being flung around the carriages on impact.

Fifteen ambulance vehicles and two helicopters were sent to the scene to help tend to the injured.

Bailey Hope, who took a video of the bloodied carriages after alighting from the train, said some of those injured had stood up ready to get out at the last stop.

The 15-year-old said it felt as though the train had sped up after leaving East Richmond and said he had heard the brakes may have failed.

"The driver had cuts all over his face," Bailey Hope said.

"There was blood everywhere."

One witness, Brett, was waiting on the platform when the accident occurred.

"[The train] came right to the end, probably at about 50 to 70 kilometres an hour, and just smashed right into the barrier," he told Fairfax Media at the scene.

"I witnessed all the people through the window; they were just flying through the air.

"I was straight onto the train as soon as I seen people were injured ??? I walked through and seen the blood everywhere. There is absolutely blood everywhere - it's a big hit.

"Inside the train is like a bloodbath. All the dividers in between the carriages are smashed, completely crushed flat on every single one, that's how fast it was going."

Linda, a Windsor mother of one whose surname has been withheld, was on her way to her first day at a new job when the crash occurred.

She was sitting down, six carriages back, when the train collided with the buffer stop and she was thrown from her seat, hitting her head in the fall.

"There were already a number of people standing up as we were approaching the platform ??? people went flying," she said.

"When I got off the train I saw people with blood on them everywhere. I saw a man lying in a carriage and he couldn't get up, his partner had a huge gash on her arm."

Sydney Trains chief executive Howard Collins said on Monday afternoon that "it is too early to speculate" as to the cause of the accident.

"The buffer stop did its job. The most modern train, the Waratah train, did its job in absorbing the impact. We are working with safety regulators," he said.

"We are looking at every piece of information we have got. We've got a lot of good information on the train - CCTV. That is with the regulator and we are co-operating."

The train suffered significant damage, and the carriages crumbled into each other.

A protection order has been placed on the train, which will see it remain in the hands of investigators until they have completed a thorough examination.

The state's Office of Transport Safety Investigations will take the lead in the inquiry into the accident, which has been put at the high end of the level of seriousness as a category two incident, allowing a deeper and more thorough investigation of the contributory factors that led to the incident.

Transport Minister Andrew Constance described it as a "very serious incident" and investigations were underway by three separate regulators including National Rail Safety and the state's Office of Transport Safety Investigations.

"Our thoughts are with staff and passengers who have been injured as a result of this train hitting the buffer stop which worked," he said.

"Our main priority obviously given today is getting to the bottom of what has occurred here, and as I said, our thoughts are with those who were injured."

Asked whether he was comfortable with the Waratah train fleet, he said there was "ongoing regulation of the railway from both the national body and in terms of the work Sydney Trains do".

"To that end, let's get to the bottom of what occurred here," he said.

With Hawkesbury Gazette and AAP