All lines have now re-opened following a fault on a train between Barnham and Horsham, according to Southern Rail.
Southern said that, due to a fault on a train, the line from Barnham towards Horsham was blocked in the Billingshurst area and trains were unable to progress towards Horsham.
The train provider has now revealed the cause of the problem.
A spokesman said: "Southern were advised by the on board staff on the 06:10 Southampton Central to Horsham that a fault had developed on the train in the Billingshurst area.
"The problem was identified as a ballast bag being wrapped around the rear train axle. As a safety precaution, the train was only able to move at a speed of 10mph, while the bag was still there.
"The train had come to stand at Christs Hospital so the driver along with Network Rail quick response staff could investigate the problem further. This in turn blocked the line for short period of time."
Southern confirmed the bag has now been removed and trains are able to run as normal.
"However, due to the length of time the issue was ongoing some service may still be subject to delay and alteration," the spokesman advised.
Southern said disruption is expected until 10am.