It’s the most important infrastructure project of the next decade and the prime minister believes it can be thoroughly reviewed by a panel of experts in just a few weeks. A report giving a final verdict should be ready by around October.

We are talking about a cost–benefit analysis of the HS2 high-speed rail line and whether it can be divined in a couple of months. If only it were that easy.

Or maybe it is, if Boris Johnson has already made up his mind. A few charts with some scarily large figures might be all he needs should he want to derail the whole scheme.

Conflicting statements from Johnson make it hard to say for sure which side he is on. A speech in Birmingham included a line about going ahead even if the £57bn project costs more than £100bn in the end. The prime minister, who is not afraid of expensive infrastructure (remember Boris Island), has also put in place a panel of eight experts to assess the project that possibly – and it’s not clear from their contributions in the past – has a majority of supporters and agnostics.

This positive outlook is tempered by the appointment of Lord Berkeley, a long-time critic of HS2, to be co-author of the final report. Having Andrew Gilligan, the journalist and self-taught transport expert, knocking around No 10 as an adviser is another potential sign of HS2’s imminent death. Gilligan tweeted earlier this year that it was a “disastrous scheme”.

But let’s say the PM is genuine in wanting an independent review, one that assesses with equanimity and even-handedness the pros and cons of plans to tunnel through the Chiltern Hills from London to Birmingham and then on to Manchester with a major spur off to Leeds.

The panel will hear that the HS2 board has done its best to cover up a long list of unhelpful forecasts, from projections of passenger numbers to calculations of tunnelling costs. There will be accusations to consider, in particular whether millions of pounds was spent paying off whistleblowers, possibly with non-disclosure agreements attached.

A queue will form outside the committee room door ready to explain that alternative routes are available and viable, that a cycle lane should have been included in the brief, and that areas of outstanding natural beauty will be ruined. Others will remind the panel that Japanese high-speed-train makers told the government to start building from Manchester, or at least Birmingham, to London and not the other way round, in order to provide jobs and garner support in the regions.

No doubt the panellists will hold their heads in their hands when they conclude that HS2 executives did all they could to buy up land, demolish buildings and point the entire transport industry in the same direction as quickly as possible, spending billions of pounds along the way, to make the project unassailable.

Emotion, though, doesn’t come into it. Ministers’ hands are tied because HS2 has been 15 years in the making and is the best compromise option the transport ministry and its advisers could come up with for improving the UK’s transport system. It is underpinned by an act of parliament that would take another decade or more to replace if the panel were to support an alternative route.

Berkeley, a Labour peer and former civil engineer who is on the board of the European Rail Freight Association, is a champion of improved rail capacity, especially for freight. He also wants greater competition to bring down prices. That tells us that he is more likely to recommend a reduction in speed to save costs, and curtail the extent of the network.

Douglas Oakervee, a retired engineer who supported Boris Island, is the other expert called on by Johnson to write the final report. He was a supporter, though he could be more savage in his assessment of cost overruns.

Yet this is a once-in-a-generation project. And without it, HS3 from Liverpool to Hull, which is championed by many despite having dubious claims for its regional benefits, will never get off the ground. The scheme needs to go ahead in full.

If it is to be half-baked, for instance terminating at Crewe and in west London, not Euston, then better to start again.

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