Metal bar pierces, derails Sydney train

Several hundred commuters have been evacuated from Sydney trains after a metal bar speared through a train floor, causing it to derail.

About 700 commuters were evacuated from a train on Wednesday at 5.30pm (AEDT) after two wheels came off the track near Edgecliff Station.

Sydney Trains chief executive officer Howard Collins said the driver noticed the eight-carriage train was sluggish before it stopped.

Mr Collins said a customer saw a metal bar come through the floor.

"A lady was a bit shocked this metal pole was near her and she was treated for a bit of shock afterwards," he told AAP.

Two wheels on the train derailed but no one was injured.

Passengers were evacuated by police and rail staff to another train.

Hundreds of other commuters had to be evacuated after their trains were trapped in the area because of the derailment.

The passengers, from a train near Edgecliff and another at Bondi Junction, were placed on buses.

Mr Collins said there was no "obvious cause" behind the incident and the train may be out of service for the Thursday morning peak hour.

"If it's still out of service it will have a big impact not just for customers from Bondi but on the Illawarra line," he said.

Mr Collins said Sydney Trains was working with an independent safety regulator to assess what happened.

The T4 Eastern Suburbs and Illawarra Line is partially closed between Martin Place and Bondi in both directions. Buses are replacing trains between Bondi Junction and the city centre.