But it is becoming synonymous with political football, disappointment, delays and spiralling costs.

The latest blow to the project, as Sky News understands, is the government's intention to axe the rail plans linking Birmingham to Manchester.

This would shrink the route significantly, with passengers only able to go between a station in a west London suburb to Birmingham - rather than from London Euston to Manchester.

Downing Street denies a final decision has been made, but a spokesperson did not set out which details were wrong and did not deny that a Cabinet meeting would be arranged.

Interventions from five regional Labour mayors and numerous northern businesses have done their best to retain the project in full, but it now appears HS2 will not be fulfilled as originally planned.

What is HS2 and why were the plans made?

HS2 was supposed to form 330 miles of high-speed rail network, initially intended to link London and the West Midlands, stretching to Birmingham, with a further phase extending to Crewe, Manchester and Leeds in the North.

It was first mooted by the Labour government in 2009 and it was hoped the increase in capacity would be a boost for connectivity between major cities.

Supporters said it would enable intercity trains currently operated by Avanti West Coast to be taken off the West Coast Main Line, creating more space for stopping services and freight trains.

Another improvement would be reduced journey times.

HS2's inception followed the development of HS1, the high-speed line between London and Kent connecting the UK to routes on the European continent.

The aim was to run 18 trains an hour in each direction to and from London - at speeds of up to 224mph - compared to between two and six an hour on Europe's high-speed railways.

Why is the Manchester leg set to be axed?

The project has been beset by delays and rising costs, with some estimates now putting the price tag at more than £180bn, a figure that's continuously risen from the 2019 estimate.

In 2019 costs were put at around £100bn.

According to the Independent, cost estimate revealed ditching the northern phase could save up to £34bn.

How much was it supposed to cost?

The original bill - at 2009 prices - was supposed to be £37.5bn.

At the time of the 2010 election, when David Cameron said his government would publish plans for a high-speed rail, £20bn was committed for rail infrastructure.

By January 2012, when the broad route of the proposed scheme was in place, this had risen to £32.6bn.

In June 2013, the coalition government increased the overall cost to £42.6bn and in November 2015, when the figures were updated, in line with inflation, to £55.7bn.

The Department for Transport's latest estimate in 2021 had spiralled even higher, to between £72bn and £98bn.

But Lord Berkeley, former deputy chairman of the government's independent review into the project, said it could climb to £107bn.

As of June, the total spent so far was £24.7bn (at 2019 prices).

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What was the route going to be?

Stations on the first phase of the line were meant to be London Euston, Old Oak Common in west London, Birmingham interchange and Birmingham Curzon Street.

There are, however, concerns the London Euston station terminus will be axed as it's been put on pause due to spiralling costs.

The line will instead end in Old Oak Common, requiring passengers to get the Elizabeth Line to central London.

This means it could be more than a decade before high-speed services stop at Euston, with passengers expected instead to travel for half an hour on the Elizabeth Line.

The second phase was meant to see trains head northwest to Manchester Airport and Manchester Piccadilly, or use existing lines via Wigan, Crewe and Stafford.

The route had also been planned to go northeast from Birmingham towards the East Midlands Hub at Toton.

From there, before the eastern extension was cancelled, the trains were due to continue on the HS2 line to Leeds, with others diverging onto existing lines via Chesterfield and York.

The original plans were:

• Phase 1: London Euston to Birmingham Curzon Street, with intermediate stations at Old Oak Common in the western suburbs and at Birmingham Airport.
• Phase 2a: To extend the line from Fradley in the West Midlands to Crewe in Cheshire.
• Phase 2b: Comprised of an eastern leg from the West Midlands to the East Midlands and a western leg from Crewe to Manchester.

How has it changed?

The eastern leg between Birmingham and Leeds was reduced to a spur line that is due to end in the East Midlands.

In March, it was confirmed construction between Birmingham and Crewe would be delayed by two years and that services may not enter central London until the 2040s.

Transport Secretary Mark Harper announced work at Euston would be paused for two years as costs were forecast to almost double to £4.8 billion.

The pause means Old Oak Common, in the capital's western suburbs, will be the railway's only London station when services to and from Birmingham Curzon Street begin between 2029 and 2033.

And the route between Birmingham and Manchester - including via Crewe - is set to be scrapped.

What will replace the Birmingham-Manchester line?

Sky News understands the Department of Transport (DfT) has worked up a package of alternative projects - rail, bus and road schemes - which could be funded from money saved by scrapping the Manchester to Birmingham leg of the project.

An expanded Northern Powerhouse Rail project linking northern cities and fresh cash for potholes and bus routes could be announced to soften the blow.

Where did it all begin?

In 2009, under Labour transport secretary Geoff Hoon, the government set up a company, HS2 Ltd, to look at proposals for a new high-speed line.

The following year, the Department for Transport (DfT) set out plans for a Y-shaped network connecting London and the cities in the North.

Later, under the Conservative-Liberal Democrat coalition, it was confirmed that the line would be built in two phases.

Phase 1 would run from London to the West Midlands, beginning in 2026. That's been pushed back to between 2029 and 2033. Euston Station is not due to open until 2035.

Phase 2, extending from the West Midlands to cities in the North, was meant to start in 2032-33. But that was moved to any time from 2034 and 2041, before decisions to scrap it.

Why is HS2 so behind schedule?

By July 2019, the government accepted that the timetable was no longer feasible and has continued to change opening times.

Reasons for the delay included a year spent revising cost and schedule estimates for phase 1 and more time being needed for construction at various sites.

In August 2019, the government announced an independent review of the programme to advise on whether to proceed.

And in March of this year the government announced more construction would be delayed by two years to save money.

The COVID-19 pandemic probably didn't help with progress, either.

In a written ministerial statement earlier this year, Transport Secretary Mark Harper said the government is "prioritising HS2's initial services" between Old Oak Common in west London and Birmingham Curzon Street.

Why have the costs risen so much?

In one word: inflation.

Mr Harper already said earlier this year, "we have seen significant inflationary pressure and increased project costs, and so we will rephase construction by two years, with an aim to deliver high-speed services to Crewe and the North West as soon as possible after accounting for the delay in construction".

A report in January 2020 by the National Audit Office (NAO) - the spending watchdog - said HS2 Ltd had not accounted for the level of uncertainty and risk in the plans.

It used a method for calculating how much extra might be needed "that was not appropriate for a programme at such an early stage of development".

Among the factors causing higher costs were commitments to increase the length of tunnelling and to erect noise barriers.

The NAO said the government and HS2 Ltd had "not adequately managed risks to taxpayer money".

More money was needed for building bridges, tunnels and stations than first thought.

Complex issues involving the discovery of asbestos and archaeological remains, and the need to divert more gas and power lines than expected, have caused problems too.

More money was also needed to buy properties to make way for the rail line.

Even after these have been accounted for, there is uncertainty around the cost of extending London's Euston station to accommodate the high-speed trains.