Sydney Trains says a motor vehicle that crashed through a fence and into a signal box in the south-west in the early hours of the morning is causing train delays on some lines.

Crews are on the site to restore the services but expect delays on the T8 Airport and South Line.

"Repairs are still underway; frequent services are operating with delays to some services. Please continue to listen to announcements and use real-time apps for specific service information. Please reach out if you require any travel assistance this morning," it said on Twitter.

Sydney Trains says the incident may also affect the T3 Bankstown Line and the T2  Inner West & Leppington Line.

Sydney Trains has also said it has buses “in strategic locations that will be used if necessary” but did not specify where.

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