Millions faced delays and disruption on roads and railways over the chilly Easter weekend, rail operators and motoring groups said on Friday.
Network Rail's Chief Executive Iain Coucher said four days of engineering work, involving 6,000 people and costing 75 million pounds, would affect some of the busiest lines.
"We know our Easter improvement programme impacts on passengers but the demands on our rail network are growing and we need to invest to deliver a better service," he said.
"Our challenge is to do this invisibly - to get in, rip up the old railway, put in a new, better one and then get out on time while causing the least amount of disruption."
Delays are expected on parts of the West Coast Main Line, which links London with northwest England and Scotland, and parts of the East Coast Main Line, which goes from London to Edinburgh.
London's Clapham Junction station, one of the country's busiest, and Liverpool Street will also be affected. Details of the works are online at: www.networkrail.co.uk/aspx/4615.aspx.
Motoring organisations warned of the "worst ever Easter congestion", with 16 million cars taking to the roads.
The RAC said the worst jams are expected on these routes:
* The M5 between Bristol and the West Country
* The M4 between London and the M4/M5 interchange in Gloucestershire
* All roads connected to the M25 around London
* The M1 from junction 25 in Nottingham to Junction 29 at Chesterfield.
* The M6 through the West Midlands
* Roads from Lancashire to North Wales and the Lake District
RAC spokesman Neil Thompson said: "This Easter's getaway is expected to be more intense as motorists shun foreign travel in favour of domestic driving holidays."
The Highways Agency warned drivers to take extra care after forecasts of rain, high winds and even snow in some areas.
A spokesman said two-thirds of roadworks have been lifted for the holidays and a fleet of gritting lorries was on stand-by to keep main roads clear.
Airport operator BAA said more than two 2 million passengers will pass through Heathrow, Gatwick, Stansted, Southampton, Aberdeen, Edinburgh and Glasgow.
In London, several London Underground lines - including the Jubilee, Metropolitan and District lines - were partially closed until Tuesday for engineering works. The entire Circle Line was shut.
The Met Office said Good Friday would see a wintry blast, with rain, sleet and high winds across much of the country.
Temperatures will range from a chilly 2 degrees Celsius (36 Fahrenheit) in the east to 9C (48F) in the southwest. Saturday will be cold and windy, but with some sunny spells.
