Fifteen ambulance vehicles were deployed to the scene, together with two rescue helicopters, with the patients taken to four hospitals in the region.
Paramedics who attended the scene treated some of the injured in a nearby park, police confirmed.
At least twelve people have been injured after a train failed to brake and hit a buffer wall at Richmond Station, in Sydney's north west, just before 10am.
"I just thought, 'What the hell was that?'"
Sydney Trains chief Howard Collins said it was too early to ascertain the cause of the accident.
"Inside the train is like a bloodbath".
It is understood that a lot of people were standing at the time of the impact, as they were preparing to alight the train.
James McCarthy Suffers Horror Double Leg Break
But the West Brom ace accidentally kicked the back of McCarthy's leg - leaving the Republic of Ireland global stricken on the floor.
Days of high-stakes negotiations are proving fruitless with the rail union and government still "miles away" from a deal that will see a Sydney-wide train strike cancelled.
The train suffered significant damage, and the carriages crumbled into each other.
"We saw the part that connects to the cables came off but then went back on again".
"Safety is our number one priority and we will now work to determine what caused the incident", said Transport and Infrastructure Minister Andrew Constance.
The Australian Transport Safety Bureau was helping the NSW Office of Transport Safety Investigations to piece together what happened.
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".
He said it appeared that those injured had suffered mostly "bumps and bruises", suggesting that the train had hit the buffer at a low speed.