A broken down train between Gatwick Airport, Lewes and Brighton is causing delays on the railways.
A Southern Rail spokesman said: "We received reports that the 4.38pm Haywards Heath to Littlehampton service developed a fault while at Haywards Heath.
"Attempts were made between the driver and fleet control to rectify the issue, however a secondary fault had developed and the train had to be taken out of service, awaiting a rescue train to tow the faulty unit to depot.
"When a train fault occurs, the driver will contact the train technicians and will be guided through a 10 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. This is likely to have an immediate knock on effect, particularly in busy locations across our network, as well as on later services in the day.
"The 4.11pm service between Ore and Victoria, will terminate at Haywards Heath and then be used to transport the train to the depot at Lovers Walk in Brighton. Until the service can be moved, services are likely to be delayed, altered or cancelled at short notice due to the congestion that has built up in the Haywards Heath area."
Train services running to and from Gatwick Airport, Lewes and Brighton will be cancelled, delayed by up to 15 minutes or revised. Disruption is expected until 8.30pm, Southern said.