Disruptions to service at Watford, Bletchley and Wembley since the New Year caused delays to the WCML's new timetable, which has significant reductions ito journey times from London via the North West to Scotland.
However, a Network Rail spokesperson said that the problems at all three locations were indpendent of work to upgrade the line, Britain's busiest.
At Watford the spokesperson said metal fatigue had caused a screw to fail.
At Wembley, the spokesperson said a fibreglass rod had been installed incorrectly, and at Blethchly an electrical wire had snapped.
Chief executive of Passenger Focus Anthony Smith, said: "The last few days have seen completely unacceptable performance.
"Billions of pounds have been spent upgrading the line. New trains have been bought. Endless assurances have been given that the track work is nearly finished and passengers could rely on the new improved timetable. Yet as ticket prices have gone up the overhead wires have come down.
"Passengers will want urgent assurances that this series of problems can be quickly fixed. Without these assurances it would be very unwise to proceed with the final stage of the timetable expansion at the end of the month. Yet again, passengers are being asked to be patient.
"In the meantime, we passengers must complain and send a clear message to the industry – you cannot treat us like this and we want our money back," he said.