Train travellers will once again face delays and cancellations because of engineering work on some of the country's biggest lines over Christmas.
Network Rail says the upgrades are part of a record-breaking £200m investment programme that begins after the last train runs on Christmas Eve.
More than 11,000 engineers will be working across the holidays.
New platforms, flyovers, junctions and station facilities will make the network more reliable for the new year.
About 90% of services will run normally, with most of the work done on Christmas Day and Boxing Day. But the work will still mean some people planning to travel by train over the holidays are going to be hit by problems.
Network Rail says only four pieces of work will extend beyond Boxing Day:
- Work around Reading restricting services into and out of Paddington (until Monday 5 January)
- Work just outside King's Cross restricts services on Saturday 27 and Sunday 28 (back to normal on Monday 29)
- Work around Watford impacting services into and out of Euston on Saturday 27 and Sunday 28 (back to normal for 29)
- Partial closure of London Bridge (until Monday 5 January).
They also point out that they have planned alternative train services and adjacent line opening to enable most services to run where the work extends beyond Boxing Day.
Analysis: Richard Westcott, BBC transport correspondent
The best thing to do if you are travelling is to put your journey into this link.
Of course, all this assumes we'll have decent weather. Remember all those storms last year? It's harder to free up engineers to upgrade a line when they are busy sorting fallen trees and flood-drenched mudslides. Let's hope it's nice.
Christmas is often a frustrating time of year to travel, but the reality is that they want to get a lot of the work done while the network is quieter. Trains are less than half full over the holidays, carrying two million passengers a day, compared with four-and-a-half million on a normal work day.
And you should notice the difference. Two new platforms will open and new track will be laid at London Bridge. They're finishing a viaduct at Reading that will unblock a bottleneck and a new signalling system should speed things up around Watford.
Our creaking Victorian network is so busy that's it's incredibly hard to fix and upgrade things during the year.
As one train company boss told me recently: "In other countries, they tend to just shut lines entirely while they carry out the work. We could never do that here."
Network Rail's chief executive, Mark Carne, is getting his apology in early.
"I recognise and apologise for the fact that some of our essential improvement work will disrupt people's travel plans," he said.
"I know that passengers can be frustrated by disruption, but people appreciate that carrying out this work at a time when the railway is less busy reduces overall disruption."