The Wales and Borders rail service should have been taken over by the state, trade unions and Plaid Cymru have said.
The Welsh Government has announced France's Keolis and Spanish-owned Amey will take over from Arriva in October.
The Rail, Maritime and Transport (RMT) union said privatisation was being enforced on Wales while the East Coast franchise had been nationalised again.
Meanwhile Tory AM Russell George called for "quick wins" for commuters.
The Welsh Government - which has taken control of the franchise from the UK government - had wanted a change in legislation to allow them to allow public bodies to act as franchises.
But economy secretary Ken Skates told WalesOnline in 2016 that UK government rules prevented this.
Plaid argues that if Welsh Government had wanted the powers Labour should not have supported the Wales Bill 2017 which did not fully devolve the railways, and that Labour had promised a not-for-profit railway in two manifestos.
RMT general secretary Mick Cash said the union's policy was for "a national integrated railway under public ownership".
"The Welsh Government has made it clear that this is their aspiration as well if they did not have to work under the pro-privatisation legislative straitjacket imposed by the UK Government," he added.
"The fact that rail privatisation is being enforced on the people of Wales by Whitehall is even more galling when the Tories have been forced to nationalise East Coast and the Westminster cross-party Public Accounts Committee have said the rail franchising model is broken."
Keolis is part-owned by French state railway operator SNCF, and takes over from Arriva Trains Wales, an arm of the German state railway Deutsche Bahn.
Manuel Cortes, general secretary of the Transport Salaried Staffs Association, said: "Instead of the Welsh taxpayers subsidising the the railways of Berlin and Bonn, they'll be contributing to the railways of Paris and Provence."
Plaid Cymru's transport spokesman Adam Price said the railways "must be brought into public hands and the Labour government must put passengers before profit".
Mr Price said: "The bidding process has been flawed from start to finish - this extends to the nature of the announcement, a one-page written statement for a £5bn project just before recess is inexcusable.
"The government hasn't even published its Invitation to Tender which it must do today so we know exactly what they asked for."
Mr Price said the Welsh Government must explain "why it has appointed two international mega-companies to run our railways for the next 15 years".
Announcing the decision, Economy Secretary Ken Skates said: "Throughout the procurement process we have prioritised investment in the quality of trains, stations and services for the Wales and Borders Rail Service and South Wales Metro."
He later told the Senedd it was too early to start "unwrapping the presents" in terms of revealing what the preferred bidder had promised for the service, due to the statutory 10-day standstill period which allowed the unsuccessful bidder an opportunity to challenge the decision.
Mr Skates said he also had "every confidence" in the Transport for Wales team who advised on the decision, despite Conservative criticism of one official's actions over the Circuit of Wales race track project in a previous role.
The UK government has been asked to comment on the matter of allowing the state to run a rail franchise.
Russell George, Welsh Conservative infrastructure spokesman, expressed concerns over the Welsh Government's "lamentable record delivering major projects in Wales".
He said: "We can only hope for the sake of millions of passengers across the country that lessons have been learned.
"As promised by the cabinet secretary last year, commuters will be expecting immediate improvements - quick wins - in their daily service."
Mr George called for passenger priorities to be "at the heart" of the new franchise, including reliability, sufficient seating and scope to handle expanding passenger numbers over time.
UKIP AM David Rowlands congratulated the winner of the contract and praised the Welsh Government's Transport for Wales arm for securing what appeared to be an "above expectations" tender.
"What we need to know now is when will shovels be turning turf," he said.
Mr Rowlands said he supported the renationalisation of the railways, saying he resented the fact that foreign companies "are allowed to come in and purchase these franchises".
However, he added: "I don't think we can blame the Welsh Government for that."
A Department for Transport spokesman said: "Devolving responsibility of the Wales and Borders franchise is a positive step and means the operator will be accountable to the Welsh Government."