Eighty metres of live cable were stolen from the trackside just north of Retford station in Nottinghamshire late on Sunday night, with the theft discovered the following day.
Detective Constable Bob Hunt of British Transport Police said it was believed the people responsible gained access to the railway line via nearby Westfield Road.
He said: “The theft caused major disruption on the East Coast Main Line on Monday morning as the cable connected vital track circuits and signalling equipment.
“It caused 2,822 minutes of delay, affecting 143 trains.
“Six train services were fully cancelled with a further six partially cancelled. The total cost of the disruption to the rail industry was £440,783.
“While rates of metal and cable theft from the railway have fallen significantly in recent years, we continue to work closely with the rail industry to bring offenders to justice and reduce disruption for passengers.”
Mr Hunt said police were keen to speak to anyone who saw anything suspicious around Retford station late on Sunday evening or early on Monday morning.
Anyone with information should contact British Transport Policeon 0800 40 50 40, or text 61016, quoting reference P/B6 of 17/04/2015, or Crimestoppers on 0800 555 111.
Fears emerged this week that Castleford, a town which saw more than a tenth of all metal thefts from the electricity network in Yorkshire and the North East last year, is being targeted by an organised crime group.