Cardiff City are making their first visit to Wembley in 81 years
Cardiff City fans have been urged to book trains in advance as they prepare for their FA Cup semi-final on Sunday.
Extra trains and coaches will travel on Sunday to London for the club's first game at Wembley in 81 years.
But train operator First Great Western has warned that services will be very busy and warned supporters to buy tickets early to be sure of a place.
There was good news, however, for fans when a planned Tube strike due to start on Sunday evening was called off.
The exodus of Cardiff fans from south Wales is beginning for Sunday's game.
The Bluebirds hope to progress to their first final since 1927, when they beat Arsenal to become the first Welsh club to win the FA Cup.
Hundreds of coaches and minibuses will also join the convoy over the weekend.
First Great Western is running three extra trains to and from London Paddington to help carry the thousands of supporters.
The company said no alcohol will be allowed on board on any service between south Wales and London or on the return journeys.
Cardiff players celebrate in their quarter-final against Middlesbrough
Engineering work in the Severn Tunnel means journeys will be 45 minutes slower than usual.
It will mean trains will travel through Gloucester and take between three and three-and-a-half hours to reach London.
Network Rail said the works, which it had been planning for 18 months, were essential.
From Paddington, fans can take a direct London Underground service to Wembley Central using the Bakerloo line, or to Wembley Park station via Baker Street.
Fears that a Tube strike could disrupt fans' travel plans have ended after a proposed strike was called off on Thursday.
If industrial action had gone ahead from 1830 BST on Sunday, it would have caused Cardiff and Barnsley fans particular problems as the game will kick off at 1600 BST.
Bus and overland rail services should still be running as normal.
National Express has two special coaches going directly to the stadium itself and hundreds of coaches and minibuses have been chartered by fans from across south Wales.
Gwyn Davies from the Valley Rams supporters' club said it had booked between 80 - 90 coaches.
Cardiff City Supporters' Club is taking another 70 and many more are expected to be taking minibuses independently.
Wembley Stadium said it was a "public transport destination" and parking at the stadium was very limited.
Spaces in the car park had to be booked 3 days in advance, and this rule also applies to blue badge holders travelling to the game.
On event days, Brent Council has restricted parking in the area to resident permit holders only.