A train has killed a cow which wandered on to the line this morning.
The collision caused delays and cancellations to rail services across Sussex and serving the capital.
Southern said passengers should expect delays and alterations following the incident near Polegate Station.
After the cow was hit, several services were badly affected.
Several messages on Twitter alluded to "animals" in the plural sense, though it is not known yet how many were involved.
The London Victoria to Hastings train terminated at Haywards Heath.
Other services from Ashford International to Brighton, Haywards Heath to Eastbourne and Eastbourne to London Bridge were terminated early to avoid the scene.
The London Victoria to Ore train was diverted to terminate in Brighton.
Brighton to Eastbourne and Brighton to Ore services were cancelled.
Network Rail says Polegate staff were on site working to resolve the problem and apologised for delays.
It is believed Network Rail know where the cow came from.
A Southern Rail spokeswoman said: "At 7.10am this morning, a train hit a cow on the line near to Polegate Station.
"This resulted in the line being shut and delays of up to 20 minutes for passengers using this route."