Rail operator Virgin Trains will add two standard carriages to each of its Pendolino trains from next spring.
It will also introduce four new trains, following the extension of its West Coast franchise until December 2012.
Earlier this year the Department for Transport said it would delay the start of a new 14-year franchise on the West Coast line until next December, to allow ministers to consult on the terms of the agreement.
As a result of the extension, Virgin says it will increase capacity on the line from April 2012, by adding a total capacity of 150 seats to each of its 31 Pendolino trains.
The group says it also plans to introduce four new trains to its fleet, each adding around 600 seats to its service.
Although Virgin will begin introducing the new carriages from April, it said that “work on introducing longer and new trains is not due to complete until December 2012, with the full benefits not being realised until the start of the new replacement franchise starting on December 9, 2012”.
It is not yet certain which operator will take over the West Coast franchise from December 2012, with Virgin likely to be competing with three other companies for the agreement.
Virgin said that the extension deal - which covers any shortfall on DfT revenue targets through an 80 per cent revenue support agreement or any excess through an 80 per cent revenue share, will see Virgin Rail Group make a premium payment to the Government for the eight months.