The East Coast main line has been under the control of the Department for Transport since the previous private-sector operator National Express pulled out in 2009.
In 2007, another private company GNER also ceased its East Coast operation after its parent company Sea Containers ran into financial difficulties.
But the Government has signalled its intention to return the line to the private sector and has shortlisted three bidders with a new franchise due to start next year.
Today, the RMT union said new figures out this week will show that Directly Operated Railways (DOR), the company running East Coast, paid £235 million to the Government in 2013/14 - an increase of 12% on 2012/13.
The RMT added that DOR will have "paid back £1 billion to the British people since it took over in 2009".
The union added that one of the bidders for the East Coast franchise was Keolis, a company tied in with the French state railways which "proves once again that the Government are happy to have state control of our main inter-city routes as long as it's not the British state".
RMT acting general secretary Mick Cash said: "It is a national disgrace that the Government are continuing with their plans to bulldoze through the re-privatisation of the East Coast line despite the latest figures showing that it is handing massive sums back to the British people while delivering huge improvements in service and customer satisfaction.
"It is simply ludicrous to even contemplate re-privatisation when not only have there been two previous private sector failures on the East Coast route but when the public-sector rescue operation has been such a stunning success."
A Department for Transport spokesman said: "A strong private-sector partner will not only provide certainty of ownership for the East Coast franchise but will be best placed to build on the significant investment planned for the route, delivering benefits to passengers and taxpayers for years to come.
"We are currently considering the bids for the franchise, and will announce our decision later in the year. In making our decision, we will look at factors including value for money, long-term benefits for communities across the route and improved services for passengers and local businesses and not just the return to the taxpayer."