The Great Western rail franchise should be returned to public ownership, the Green Party has said.
Transport giant First Group will carry on running the service until September 2015 before bidding starts for a new long-term franchise.
The decision was made earlier this month after the Government's awarding of longer-term rail deals ran into trouble last year following problems in the tendering process for the West Coast Main Line.
Now the South West Green Party wants the contract re-nationalised when the extension expires in two years' time.
The East Coast Main Line has been publicly owned since 2009, when National Express handed back the franchise amid financial problems.
But ministers this week said the private sector will "revitalise" it as they kick-started the franchising process.
But the Green Party said it was "fully committed" to the re-nationalisation of the railways, arguing it would save taxpayers more than £1 billion a year and lead to lower fares and better services.
And Labour's new shadow transport secretary, Mary Creagh, has said she is "open" to the idea of returning all train services back to state control, saying: "We don't rule anything out."
The Green Party's sole MP, Caroline Lucas, has recently launched a Private Member's Bill in Parliament calling for the rail network to be brought back into public ownership step by step as franchises expire, or private companies break the terms of their agreements.
Greens, together with the RMT Union and campaign groups, say the success of the East Coast Main Line in public control has delivered £600 million to the Treasury, increased passenger numbers, improved efficiency and consistently scored highly in passenger satisfaction surveys.
First will pay just £32.5 million in premiums to run the route for two more years – despite having earlier walked away from a contract to run the line until 2015 to avoid payments of up to £800m to the government.
Andrew Bell, a transport campaigner and prospective Green councillor in Exeter, said: "The Green Party in the South West is calling time on the First Group. Privatised rail has been a disaster for passengers, the taxpayer and for many of those working for the private operating companies.
"The franchise model may be great for shareholders, but it's terrible for passengers who have to put up with some of the highest fares in Europe and for that they get overcrowded trains and unreliable services.
"Since privatisation, the cost of train travel has risen by 23% in real terms, and the drain on the public purse has more than doubled. In addition, these companies treat some of their employees or subcontracted employees with contempt."
He added: "We echo the RMT's call for a living wage and the ending of zero-hours contracts for all employees working for the First Great Western franchise."
Proponents of rail privatisation point to the success of booming passenger numbers, the soaring number of services and customer satisfaction at near-record highs, and contrast that with poor punctuality and dirty carriages in the days of British Rail. The new Great Western deal secures an extra two carriages on the sleeper service, improves wi-fi on long-distance services, and retains through trains to London.