The deadline has expired for rail operators who want to take over the Great Western Mainline with a new 15-year franchise.
Firms had to register their interest with the Department for Transport (DfT) via a Pre Qualification Questionnaire.
The service runs between London, the west of England, south Wales and parts of the Midlands.
Several large UK and European rail firms are said to be interested in taking over the franchise.
The DfT said it did not wish to comment on potential franchisees while the process was ongoing.
Current operator First Great Western, which was awarded the franchise in 2005, said it did not want to take up the option to extend its current contract beyond March 2013.
The firm said it wanted instead to try to negotiate a longer-term deal under new economic conditions and is expected to rebid for the franchise.
The new franchisee will be responsible for overseeing a £5bn investment programme of rail electrification and inter-city service improvements on the Great Western rail route.
The 10-year improvement plan, the largest revamp of the Great Western railway since it was built by Isambard Kingdom Brunel 175 years ago, began last July.