On Monday, the track operator will be answerable to parliament – to a chorus of executives and MPs playing the change down

It is June 2029, and the last train run by a private company rolls into platform 28 of the new Euston mega-station. The moment is marked only by a low-key ceremony with a handful of managers and long-retired staff, since few in the railways want to reignite the long-bubbling political row over what only the oldest members of the House of Lords still call "a return to the bad old days of British Rail".

Yet, as Labour ministers are fond of pointing out, the first moment in the long wave of renationalisations started back in September 2014 – the final months of the Cameron government – when Network Rail came back under state control.

Or did it? The semantics and equivocation over Network Rail's status already resembles a saga to equal Boris Johnson on his ambitions. The government insists its impending reclassification is a "statistical decision" – not (dread word) nationalisation.

Network Rail is more relaxed over what people choose to call it. The fact is that from Monday, the track operator moves from being nominally in the private sector to a public sector organisation, answerable to parliament and with a ballooning £34bn debt now on the national balance sheet.

Claims that ministers are seizing control are dismissed by both Network Rail and the coalition. Yet on Friday Network Rail's 41 public members – to whom the board is theoretically accountable – vote on allowing the secretary of state to hire and fire the chair of the track operator, have a greater say over the board and in effect control the appointment of those public members.

One member, Lord Berkeley, who chairs the Rail Freight Group, says: "I think it's a very big deal. A special member will appoint a special director and will fix the director's bonuses, executive pay and approve strategies. If that isn't state control, what is?"

At the transport department, the official line holds firm. A spokesman says: "The classification of Network Rail is an independent statistical decision by the Office of National Statistics. It does not change the industry structure or affect the day-to-day operations of the rail network. The ONS's decision has no effect on fares, performance, punctuality, safety or timetables."

At Network Rail, as befits an organisation the government wants to insist is at arm's length, the official word is slightly different: "For passengers and station users, this statistical change will mean little to their journeys. We recognise that, for Network Rail, it will mean greater accountability and transparency to parliament, taxpayers and fare payers, who rightly deserve to better understand the value of their significant investment in Britain's booming railway."

Network rail WEB 2908

For all they play down its significance, the reclassification is something both parties have struggled to avoid. One adviser to the last government recalls annual edicts from the Treasury warning the transport secretary that, however desperately they might wish to condemn Network Rail directors' bonuses, any hint of exercising direct political control would lead to the ONS calling out the taxpayer-funded track operator as a public body – and keeping its ever-increasing borrowing off the public balance sheet – was a concern well before the 2010 election made government debt a defining issue.

Hence, later, the tortuous solution employed by the then transport secretary, Justine Greening, in the face of bonus uproar in 2012: she said that as an ordinary public member of Network Rail, she would vote against the awards, rather than demanding action as a minister.

On the railway side, one well placed Network Rail source admits: "The fear is that there will be real scrutiny of spending." He compares current arrangements to how franchising has helped investment in the railway: the befuddling flow of finance might mask who ultimately pays, but bids to run train lines that come with promises or conditions of upgraded stations, rolling stock and facilities come at one more remove from the Treasury – potentially giving first the DfT and the rail industry a greater degree of effective public spending than it might have if the competing claims of wifi on trains or a new hospital were directly linked.

The £38.5bn core budget for the next five years is approved and the government insists there are no plans to curb investment. But those fears have escalated since a summer row between chief executive Mark Carne and an unsuspecting Patrick McLoughlin, transport secretary, over an additional £1.5bn needed for electrification projects, which has resulted, one source claims, in "armies of civil servants crawling over the budget" even before the track operator officially doffs its cap to McLoughlin.

Until now, the answer would have been for Network Rail to simply borrow: its spending on the never-never has largely escaped public attention, despite the industry jocularly speaking of the "Network Rail credit card".

That card continually accrues debt – with minimal repayments – by borrowing against its regulated asset base or the railways' overall value as bigger stations are built and new track laid.

It is a bit like someone constantly drawing down cash against their house, spending it on improvements and borrowing yet more on the basis that it could be sold for more, should someone fancy purchasing a 17-room mansion with an ugly side-return and triple-glazed conservatory. Meanwhile the mortgage is never paid down.

As Roger Ford, editor of Modern Railways, puts it: "When government authorises an enhancement – say the Midland main line electrification – it does so on the basis that Network Rail will borrow to pay for the work and that taxpayers yet unborn will stump up through the RAB mechanism to pay the interest on those loans."

The rail regulator, ORR, last year gently suggested that the government might like to have a peek at Network Rail's funding mechanisms as it forecast the debt to pass £50bn by the end of the decade. But, says Daniel Brown, director of strategy and policy at ORR: "It's not a significant amount against the whole of public debt." He believes the reclassification is a positive move: "This will enhance transparency and accountability. They will now borrow money through the Treasury rather than from the City – it's slightly cheaper."

Others are more sceptical, especially as the track operator appears likely to be excluded from Freedom of Information (FoI) legislation. Manuel Cortes, leader of the TSSA transport workers' union, which is campaigning to ensure FoI applies, says it will mean "a public sector firm, carrying a £30bn debt mountain and with a five-year spending budget of £38bn, being removed from financial scrutiny by the very people who are paying all these vast bills – the UK taxpayers."

Despite insistence all round that it is business as usual, the row highlights that the move is politically unpredictable. Stephen Joseph, chief executive of the Campaign for Better Transport, says: "I do think it will give government a lot more opportunity to interfere – for good or ill – than before. There is some scepticism within government that Network Rail is as efficient as it should be. I'm sure there will be pressure on them to address that."

He points to changes in personnel at the DfT to "manage the relationship", with a new rail executive established: "In the short term things won't much change, but in the longer term there is potential for a lot more to change, whoever wins the election."