First ScotRail will no longer run the six-nights- a-week overnight services after ministers handed it to Serco from April 2015 in a deal worth up to £800 million.
The company - headed by the prominent businessman Rupert Soames, a vocal critic of independence - has insisted it would improve the service, which has been criticised for its ageing decor and rolling stock.
Customers will benefit from a fleet of new carriages within four years, including ensuite berths, "Pod" flatbeds and a new dining car.
Other promised improvements include a new booking website, and a broader range of fares and earlier boarding.
However, the acting general secretary of the RMT, Mick Cash, suggested the "logical option" of public ownership of the service had not been considered. He added: "Quite frankly, with their appalling list of failures in the UK and globally, they should never have even been considered as contenders for the Scottish sleeper service."
"Serco is a company that has a reputation for promising the earth and delivering quite the opposite as they seek to maximise profits and sweat their assets for every single penny piece."
Serco and security firm G4S are being investigated by the Serious Fraud Office (SFO) and have agreed to repay a total of more than £180m after it emerged that they had overcharged the taxpayer for electronic tagging.
He said the RMT was "seriously concerned" at reports of financial problems with the firm and the investigation.
Manuel Cortes, leader of the TSSA rail union, said: "This decision flies in the face of everything Alex Salmond has been saying about an independent, financially strong Scotland.
"He has taken the contract away from a Perth-based Scottish firm and handed it to an English-based firm, which will be given a £60m subsidy to build new trains in Spain. How does that create any new jobs in his dreamland of an independent Scotland?"
Mick Whelan, general secretary of train drivers' union Aslef, said he would be seeking reassurances on behalf of its members.
Serco said it would not invoke a seven-year break clause if independence happens.
Transport Minister Keith Brown insisted the constitution was not a "material consideration" in the deal but confirmed the break clause had been agreed to take stock of changing economic circumstances. He said the Serco deal offered a "dramatically improved prospect at a reduced price"
Jamie Ross, business development manager at Serco, said: "There are no plans for staff reductions."