The rail service provider said a train driver reported seeing the animals on the track near Amberley earlier this afternoon.
Network Rail response staff were sent to search the area. It has now been cleared.
Trains are running at a reduced speed between Barnham and Horsham due to the incident.
Services may be cancelled or delayed by up to 15 minutes, Southern Rail said.
The disruption is expected to last until 4.45pm.
Don’t miss out on all the latest breaking news where you live.
Here are four ways you can be sure you’ll be amongst the first to know what’s going on.
1) Make our website your homepage
2) Like our Facebook page
3) Follow us on Twitter
4) Register with us by clicking on ‘sign in’ (top right corner). You can then receive our daily newsletter AND add your point of view to stories that you read here.
And do share with your family and friends - so they don’t miss out!
Always the first with your local news.
Be part of it.