Damage to overhead wires has caused major disruption on the Great Eastern railway line.
Trains are currently unable to run between Colchester and Marks Tey in Essex because of the damage which was discovered at about 04:45 BST.
Engineers are on site but passengers are being advised not to travel between Norwich and London Liverpool Street.
The repairs are likely to take several hours and passengers are being warned they face disruption all day.
Train operator Greater Anglia said: "Customers from Norwich, Diss, Stowmarket and Ipswich travelling towards London can use services via Cambridge.
"Tickets will also be accepted on Great Northern services between Cambridge and London King's Cross. Customers with tickets dated [Thursday] can travel [on Friday].
"Greater Anglia and Network Rail sincerely apologise for the disruption caused."
It is not yet known what has caused the damage.