But they warned that the operator, which plans a summer of weekend closures of sections of the West Coast Main Line in order to complete the work this year, must do all it can to minimise passenger disruption.
Bosses of the Office of Rail Regulation also said they would be "closely monitoring delivery of the plan" which will hopefully see the giant project finally finished by December 2008.
Earlier this year, Network Rail announced that it would have to close parts of the line on non-bank holiday weekends as well as bank holidays so that engineering work could take place.
This followed the disruptive engineering overrun at the new year when Network Rail failed to complete on time the Christmas holiday work at Rugby in the West Midlands.
This was one of three new year engineering overruns which led to the ORR fining Network Rail a record £14 million.
ORR access planning and performance director Michael Lee said: "It is vital that Network Rail now delivers this plan. We will closely monitor Network Rail's performance between now and December, ensuring that the new infrastructure operates reliably and is maintained safely."