Rail passengers suffered delays and disruption on Monday after cable thieves hit rush hour services into London.
Cable was cut and stolen from the line in Hertfordshire, hitting services into Liverpool Street station.
Rail staff reported the latest theft to British Transport Police, which appealed for information.
Detective Inspector Terry Pearce, of BTP's dedicated cable theft team, said: "There is a common assumption that cable theft is a victimless crime, with the only effects being felt by the railway industry. This is not the case.
"Theft of crime cable can, and does, cause significant delays and cancellations to the rail industry - as well as costing the industry millions of pounds each year.
"What thieves don't realise is that they're attempting to steal cable carrying up to 25,000 volts. Not only are they risking a court appearance, they're also risking their lives."
Bob Crow, general secretary of the Rail Maritime and Transport union said: "Yet again rail services are disrupted due to the continuing scourge of cable theft.
"RMT repeats its demand for the massive profits of the private train operators to be diverted into providing the staffing and security levels required to stop this lethal criminality on the tracks."