- 21 updates
Network Rail engineers are currently at Balcombe Tunnel, pumping water away to allow a limited train service to begin.
Passengers are, however, being warned to check before they travel as further heavy rain is due in the area.
Two other lines remain blocked in the county, with the East Grinstead line flooded at Lingfield and Dormans and the Redhill-Tonbridge route blocked at Nuffield.
Network Rail's route managing director for Sussex, Tim Robinson said, "It has been a difficult start to the day and we apologise to passengers for their disrupted journeys today.
"We are working hard to get the lines clear again as soon as possible."
Sussex Police are advising motorists to be aware of the conditions on the roads today as heavy rain fell over night, bringing major flooding issues to West and Mid Sussex.
The A23 is currently closed between Bolney and Pease Pottage, leading to extensive tailbacks.
Although diversions have been set up, many other roads in the area have been badly affected or closed by flooding.
- The A24 at Broadbridge Heath, Findon and Dial Post
- The A29 at Bury Hill, near Pulborough
- The A281 at Henfield
- The A27 east of Chichester
Southern Railway have released this statement regarding the flooding that is affecting their services:
A23 West Sussex - A23 closed in both directions between Warninglid and Pease Pottage, because of flooding. Diversion in operation - via A272, A24 and A264 to M23 at Pease Pottage. In the roadworks area. Congestion on Northbound to Hurstpierpoint and on Southbound M23 to J10a B2036 Balcombe Road.