Published: 14:09, 11 September 2020
| Updated: 14:22, 11 September 2020
The rail firm released a video showing water flooding the tracks in the Sevenoaks Tunnel while engineers carried out an inspection.
In a thread on Twitter Network Rail said: "Although it has just reopened, there will be disruption to Southeastern services for some time.
"Following the Stonehaven tragedy, we have new rules in place for these circumstances where water is pouring onto the tracks and we stopped trains running at around 10.30am until a structures inspector could examine it.
"The Sevenoaks Tunnel is more than two miles long and opened in 1868. It was built through several underground springs, which are unpredictable in their flow, and we recently completed a major project to improve the drainage of the tunnel and the lining, to make it more reliable.
"A drainage channel was laid between the tracks to carry the spring water away and without it, we would have had trouble getting rid of the water pouring through the roof.
"Trains are now running and we are investigating the source of the deluge.
"We're really sorry for the disruption today and we hope you understand why we couldn't take any chances."
The structural engineer declared it was safe to reopen the railway, which happened at around 12.40pm.
Southeastern introduced bus replacement services between Tunbridge Wells, Sevenoaks and Dunton Green while the tunnel was closed .
On August 12 three people, including the driver and a conductor, died after a passenger train derailed near Stonehaven in Scotland.
The Rail Accident Investigation Branch said the train struck a landslip covering the track after heavy rainfall.