The regulator made the request on 28 February, asking Network Rail to produce a plan showing how it would complete the work on the West Coast main line to enable significant timetable improvements.
Network Rail must now deliver this revised plan allowing the improvements to be introduced from December 2008.
But the regulator warned Network Rail about mitigating the effect of disruption caused by engineering work, and said that it would be closely monitoring delivery of the plan.
ORR Director of Access Planning and Performance Michael Lee, said: "The West Coast Main line is a key part of the national rail network, linking London, Birmingham, Manchester, Liverpool and Glasgow.
"Network Rail's planned improvements will mean better services for passengers across a wide part of the network from December this year.
"We have scrutinised this plan, making use of expert independent advice, and considering the views of those who run services, representatives of those who use them, and the government bodies that provide funding.
"It is clear that most people want to see the improvements made this year. Passengers will unfortunately face increased disruption while this work is being done, but will benefit from the new timetables in December 2008. We are very conscious of the adverse impact on rail freight, and Network Rail needs to do all it can to ease the effect of its engineering work on users of the railway.
"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," he said.