According to Southern Rail, engineers are on site and are working to repair the level crossing barrier.
The fault was first identified shortly after 8.20am.
A spokesperson said on Twitter: "Due to a fault with the level crossing barrier at Billingshurst, services between Horsham and Barnham will be running at a reduced speed through this area.
"You can continue to use your normal route for travel but please allow some extra time for your journey.
"If you are travelling from Billingshurst itself, the footbridge will allow you to cross between platforms, but you may need to use an alternative route if you are driving or cycling to the station."
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