Disruption is expected until 9pm while engineers and staff resolve the issue and make safe the area, a spokesperson for the railway company has said.
Other alterations and cancellations are also possible, so customers are urged to check before travelling this afternoon.
Some customers may need to use an alternative route to complete their journey, which may take 45 minutes longer as a result.
Tickets will be accepted for no extra cost on alternatives routes for no extra cost:
The following Stagecoach bus routes:
The following rail alternatives:
A spokesperson for Southern Railway said: "A service has developed a fault in the Collington area and is unable to move, blocking the line towards Hastings, Ore and Ashford International.
"Specialist engineers are being sent to the site to assess and fix the issue as quickly as possible. If needed, Southern may also arrange a rescue train to tow the faulty train to a depot.
"While Southern work to move the train, Southern will try to divert other services if possible. This depends on available tracks and signalling options.