Most of the injuries were cuts and bruises but at least three people have more severe injuries.

Three helicopters, 17 ambulance crews, as well as Fire and Rescue NSW and SES crews, were on the scene from about 10am. A major triage area was set up to assess patients, a Fire and Rescue spokeswoman said.

An ambulance spokesman said the patients with the worst injuries included a 31-year-old woman who had a fractured collarbone, a 77-year-old woman who had a fractured collarbone and suspected spinal injuries and a 21-year-old man who had a fractured femur. All were taken to Westmead Hospital.

The spokesman said 10 patients were given a "green card" assessment, meaning they were conscious, breathing and had minor injuries. He said they might be taken to hospital in a minibus.

Another patient was taken to Nepean Hospital, while two "green card" patients were taken to Blacktown Hospital.

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."

A woman told Fairfax Media the store in which she works near the station shook when the collision occurred.

Sydney Trains chief executive Howard Collins said the train that hit the buffer on platform two at Richmond station was a new Waratah train.

"My first thoughts are with those who have suffered some form of injury," he said.

Mr Collins said Richmond station had been closed under the guidance of safety regulators, and trains were running only as far as Clarendon, two stops before the end of the line, and replacement buses had been put on.

He was repeatedly questioned at a press conference on Monday about the likely cause of the incident but said "it is too early to speculate".

"The buffer stop did it's job. The most modern train, the Waratah train, did it's job in absorbing the impact. We are working with the 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 cooperating."

The injured Sydney Trains staff member included a cleaner who was taken to hospital suffering a head injury.

Earlier, he described it as a "most unusual occurrence" for a train to hit a rail buffer and "obviously very concerning".

Sydney Trains management had been locked in negotiations with rail union officials on Monday morning to discuss a dispute over pay and conditions.

Those talks were put off for several hours until the afternoon while senior management at Sydney Trains came to grips with the extent of the Richmond accident.

NSW Transport Minister Andrew Constance described it as a "very serious incident" and he said investigations were already underway by three separate regulators including the Office of the National Rail Safety Regulator 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, Mr Constance 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.

Witnesses told radio station 2GB the scenes were chaotic.

One caller who saw the crash said there was an "almighty bang".

He said the train came in at speed and hit the buffer and rebounded.

"There was a huge amount of dust," he said.

"It's all chaos here," another caller, Steve, told the station.

With the Hawkesbury Gazette, AAP