Southern Rail has warned that trains may be cancelled, delayed by up to 25 minutes or diverted between Balcombe and Three Bridges. Disruption is expected until 5pm.
A spokesperson said: "If you are travelling from Brighton towards Three Bridges/Gatwick Airport, please allow an extra 20-25 minutes for your journey.
"Only one line is available between Three Bridges and Haywards Heath so service will be diverted which will extend your overall journey time. Whilst this takes place, some trains will be held in station platforms."
Southern said that, when a train fault occurs, the driver will contact technicians and will be guided through a ten minute repair process.
"If they are unable to fix the fault, the train will be terminated and taken out of passenger service and returned to the depot for full safety investigation and repairs," the spokesperson said.
"This is likely to have an immediate knock on effect, particularly in busy locations across the network, as well as on later services in the day.
"In this situation, a train that was not in passenger service has broken down and requires a 'rescue train' to attach and move it."
A 'rescue train' is on it's way and Southern hopes to have the line reopened 'as soon as possible'.