Eastbourne commuters faced long delays on Thursday morning after a train hit and killed a cow that wandered onto the line.
The collision, near Polegate Station, caused many services to be cancelled, including Eastbourne to Brighton trains and those running along the coast from Brighton to Ore.
Trains from Eastbourne to Haywards Heath and London Bridge were terminated early to avoid the scene.
Network Rail said Polegate staff were on site working to resolve the problem and apologised for delays.
A Southern Rail spokeswoman said, “At 7.10am on Thursday 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.”
It is not yet known how many animals were involved in the incident.