Rail users suffered fresh delays today after another incident of cable theft, disrupting trains into London for the second time this week.
Greater Anglia services into the capital's busy Liverpool Street station were hit after the incident near Romford in Essex.
The rail operator's services were affected on Monday after thieves stole 1,200 metres of overhead cable at Cheshunt, Hertfordshire.
Meanwhile, vandals threw a shopping trolley onto the main Brighton-London line at Horley in Surrey, where it was struck by an early morning train into the capital.
The incident damaged track cables and led to delays to later services.
A Greater Anglia spokesman said: "Train services in the Romford area were subject to cancellations and alterations this morning until approximately 6.50, 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, saying the incident was reported at 3.09am 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.
"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.
"The act of stealing, or attempting to steal, cable is extremely dangerous and anyone who has any information about the incident, or who saw any suspicious activity in the area, is asked to come forward," said a police spokesman.
Network Rail said the trolley was struck by two trains and became lodged underneath the second one - the 4.17am to London from Purley, leading to trackside cable being damaged.
A spokesman said: "This reckless behaviour is unacceptable and those responsible are fortunate that the impact of their actions was not more serious.
"Network Rail fully supports British Transport Police attempts to track down these criminals so they can face the consequences of their irresponsible actions."
Police said later that a luggage trolley was thrown from a footbridge around 600 yards from Gatwick Airport station.
Detective Sergeant Bob Stokoe said: "We are currently carrying out local inquiries, including identifying forensic opportunities and speaking with witnesses. We have also requested CCTV from the station and local area.
"I cannot over-emphasise enough the dangers associated with this type of crime. It is vital that we find those responsible.
"Thankfully this time, through luck, no one was injured and no trains were damaged, but we may not be so fortunate if this happens again. Trains travel at high speeds and there is a very real risk when they strike objects on the track that they could be derailed.
"The obstruction damaged the tracks and resulted in delays to train services through the area, which could have easily delayed hundreds of passengers heading to the airport and missing their flights over the Easter holidays.
"We are urging anyone who was in or around that area to contact us if they saw anything suspicious or have any information that may help us with our inquiries."