A plan to provide thousands of extra seats a day on Britain’s busiest rail route is likely to be postponed indefinitely because Network Rail fears that there are many undiscovered defects on the overhead power lines.

More than 200,000 passengers have had journeys cancelled or disrupted on the West Coast Main Line in the past four days after a series of problems with the cables.

Network Rail has just completed a £9 billion upgrade of the route and issued a statement last month praising its own “massive achievement”. It promised that a new timetable would “herald a step change in the frequency and speed of train services”. But in the rush to complete work in Rugby and farther north, it appears to have neglected basic monitoring and maintenance of the infrastructure closer to London.

Overhead lines have collapsed three times in the past three days, with the most serious incident resulting in passengers being trapped for more than an hour on a freezing train near Wembley, northwest London, on Tuesday night. They were handed emergency blankets and chocolate, and were eventually led to safety by torchlight along the tracks.

Neither Virgin Trains nor London Midland could run any services in or out of Euston station in London during the rush hour yesterday morning.

The Times has learnt that Virgin and Network Rail are now discussing whether to postpone a big increase in capacity due on January 25, when 20 extra high-speed services were to be added to the daily timetable. The companies are also considering whether they can sustain the extra trains added on December 14, when the frequency from London to Birmingham and Manchester increased from two to three trains an hour.

Virgin had previously advised Network Rail to delay increasing capacity until May to allow time to ensure that the line could cope.

Network Rail is investigating whether the higher frequency and speed of trains contributed to the cable failures.

The company has stopped carrying out daylight inspections of the lines and does them at night instead to avoid closing tracks during the day. An industry source said that it was much harder to spot defects at night.

The £9 billion upgrade did not include overhead lines south of Milton Keynes, where there are thousands of older ceramic insulators vulnerable to cracking in cold weather.

A Virgin source said: “Network Rail has imposed a 110mph speed restriction all the way from London to Rugby on lines which should now run at 125mph. They clearly lack confidence in their own infrastructure, which is very disappointing considering how much has been spent on the West Coast line in the past decade.”

The British Chambers of Commerce estimated that the disruption had cost business £38 million over the past three days. David Frost, its director-general, said that the organisation had challenged Network Rail’s decision to bring forward the deadline for the upgrade last year. “We believe the recent disruptions have occurred because this vital project was rushed.”

Gerry Doherty, of the TSSA rail union, said: “This is the second year running that travellers have had to face new year chaos on the West Coast line despite assurances that the much delayed upgrade work had finally been finished. Passengers should be given full refunds for the past three days and [Network Rail] should explain why passengers are suffering these shambolic breakdowns after £10 billion of taxpayers’ money has been spent on this upgrade.”

Last year three big projects, including one on the West Coast line at Rugby, overran, resulting in chaos. The Office of Rail Regulation fined Network Rail a record £14 million.

Louise Ellman, chairman of the Commons Transport Select Committee, described this year’s failures as unacceptable. “Network Rail must conduct an urgent investigation into this massive breakdown and take action to restore confidence in the rail system in which so much public money has been invested,” she said.

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