Cable theft hits London commuter line from East Anglia
Early commuter services into London were disrupted after about 150ft (45m) of overhead cable was stolen.
Trains from Norfolk, Suffolk and Essex into Liverpool Street suffered delays after cable was torn down between Romford and Gidea Park stations.
Operator Greater Anglia apologised to passengers for the second time in a week but said services were now normal.
On Monday thieves stole about 4,000ft (1,200m) of overhead cable at Cheshunt in Hertfordshire.
A Greater Anglia spokesman said: "Train services in the Romford area were subject to cancellations and alterations due to damage to the overhead power lines believed to be caused by cable theft.
"We worked closely with Network Rail to restore the normal service as quickly as possible and apologise for any inconvenience caused."
British Transport Police appealed for witnesses to the latest cable theft to come forward, saying the incident was reported at 03:10 BST by a member of rail staff.
"Officers attended the scene between Gidea Park and Romford stations, alongside engineers from Network Rail, and discovered around 50 yards of copper cable had been stolen," they said.
"This theft caused considerable disruption to early-morning commuters.
"What thieves don't realise is that they can be working on live cable, carrying up to 25,000 volts, which can cause extensive burns."