Disruption on the West Coast Main Line was caused by a "rush" to upgrade the route, it has been claimed.
The British Chambers of Commerce (BCC) said the upgrade had been completed in haste, leading to a string of problems.
Services were hit for the sixth day in a row when a fallen power cable halted trains to and from London Euston.
Rail engineers worked through the night to try to prevent another day of travel chaos. Network Rail said it expected a full or near-full service on Thursday.
"This will feel like deja vu to many customers on the west coast line"
On Tuesday night overhead power cables hit a train at Kenton in north-west London, closing the line between North Wembley and Kings Langley, Hertfordshire, until mid-morning on Wednesday.
A section of track close to the site of the fallen cables was closed again at 2130 GMT to allow emergency engineering work with the aim of getting services back to normal for Thursday morning.
Late-travelling Virgin passengers are being redirected to St Pancras for trains to Luton Airport Parkway where coaches were running to Milton Keynes.
Passengers are being sent to Marylebone for trains to Birmingham.
London Midland and London Overground passengers are able to catch buses from Euston to Watford. Some London underground services are also being affected.
£38m lost
The latest power failure was the fourth incident in a week to disrupt the West Coast Main Line, which has led to 70% of all Virgin services running late since the start of the year.
BCC director general David Frost said: "Businesses have lost a staggering £38m in just three days because Network Rail have again failed to adequately deliver.
"The BCC questioned Network Rail's decision to bring forward the deadline of the west coast upgrade last year.
"We believe the recent disruptions have occurred because this vital project was rushed."
Shadow transport secretary Theresa Villiers said: "This will feel like deja vu to many customers on the west coast line.
"If there is any truth in the claim that disruptions have been caused by rushed work by Network Rail this would be a serious problem."
"This is not the best start to the new year for passengers"The latest power failure followed problems on Tuesday at Rugby in Warwickshire and Bletchley in Buckinghamshire.
Before that travellers were disrupted on Sunday and Monday as engineers worked at Watford Junction to repair damaged power lines.
And on Friday and Saturday the line was closed after a plane crashed near the tracks in Staffordshire, killing three people.
A Network Rail spokesman said a "full investigation" was being carried out into the cause of all the problems.
"At this stage there is no suggestion that the new timetable is a factor in this.
"We understand that passengers are frustrated by what's happened this week. We have worked very hard to rectify the problems."
The TSSA rail union has called for passengers to be given full refunds.
And Anthony Smith, chief executive for the national rail watchdog Passenger Focus, said: "This is not the best start to the new year for passengers and we hope that a normal service will resume as soon as possible - especially since passengers are now paying more to use the train."
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