RailCorp will launch an investigation into how a driver ran a red light causing a suburban train to derail just before the afternoon peak yesterday.

No one was injured but CityRail and emergency services freed about 150 passengers trapped in the train, and the Illawarra Line was blocked in both directions for an hour, throwing evening services across the network into chaos.

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Train derailments are not common in Sydney.

An initial investigation found that driver error caused the accident, RailCorp said in a statement last night.

The driver has been suspended from duties pending further inquiries.

"The driver and guard have been drug and alcohol tested. The alcohol tests have returned negative and the results of the drug tests take longer to process. The safety regulators have attended the site.

"As part of normal RailCorp procedure, the driver has been taken off driver duties pending further investigation."

The Minister for Transport Gladys Berejiklian said in a statement that the incident was serious and she was "relieved no one was hurt".

"I regret people were inconvenienced. Fortunately the safety system kicked in as it was supposed to.”

The front of the train came off the tracks on the Illawarra Line at 4.40pm just after the train had left Hurstville station heading from the city towards Cronulla.

"It appears the driver may have passed a red signal which activated the safety device on the track. This device caused the front bogies to derail [the bogie holds the four wheels on the front carriage of the train]. This is the standard fail safe requirement on our network," RailCorp said.

Passengers on the Illawarra Line, the South Coast Line and the Eastern Suburbs Line were directly affected, with delays of up to an hour. Buses replaced trains for passengers travelling between Central and Bondi Junction.

A spokesman for RailCorp said no one was injured and that services today would not be affected.

For more information, contact 131 500 or see @131500trains.