A train has hit a large fallen tree between Barnham and Chichester, smashing the front windscreen.
The driver aboard the service from London towards Portsmouth Harbour has said the big fir tree was blocking both lines.
No one is hurt but passengers have been advised that the train is likely to remain stranded for some time.
Driver Nick Westwood, who is based at Barnham, said he'd never had a similar incident in 16 years of driving trains.
He said: "It smashed all the front doors off and a lot of the front of the train.
"A lot of the wood is underneath the train now."
He said he got out the way of the main window while he made an emergency stop from 60 mph but the branch fortunately hit the right-hand panel.
He said: "There was little I could do, the line speed here's 75mph."
A reporter on the train has said the collision felt like the train rolled over several branches before coming to a standstill.
The train is stopped just before the A27 bypass.
Network Rail engineers and British Transport Police are now on scene and a relief train is being organised.
Passengers were due to arrive in Portsmouth at 9.05am.