HS2 costs got "carried away", says the author of the government's official review into the project.
But Douglas Oakervee, also a former HS2 chairman, said the line was still "much needed".
It comes after Boris Johnson gave the route the go-ahead but said it was a "controversial and difficult decision".
Speaking publicly for the first time since the decision, Mr Oakervee said he welcomed steps for "proper governance" of the scheme.
He said there had been an "issue" with multiple chairmen and secretaries of state being involved in HS2 since its inception.
"And they have always retained the 2015 prices which I don't think is particularly right, I think they ought to be updated all the time so both the public and Parliament are fully aware of what those prices are," he said.
HS2 is set to link London, Birmingham, Manchester and Leeds and it is hoped it will cut down passenger overcrowding and help rebalance the UK's economy.
It was first signed off by MPs in 2017, but has faced strong opposition.
It has been revealed the project could cost more than £100bn - twice its 2015 budget - and could be up to five years behind schedule.
Despite that, the Oakervee Review strongly advised against cancelling HS2, saying it would benefit the transport system and there was no "shovel-ready" alternative upgrade for the existing railways.
It recommended tighter controls on costs and better management.
On Wednesday, Mr Oakervee said the rail line "should have been done in a much more practical way".
He said the "dream" project had been allowed to continue without necessarily "working within a fixed budget."
In an interview with BBC Midlands Today, he suggested using the private sector to develop new HS2 stations in a bid to cut costs.
"I think Curzon Street would benefit from a lot of private sector involvement, rather than building a station that will win many architectural prizes, at what cost," he said.
He also said China's state railway CCRC had been "bold" in suggesting it could build the line in just five years and at a much lower cost, as the country operated very "different" legislation to the UK.
Prime Minister Boris Johnson said HS2 and "dramatic improvements" to local transport would "shift this country's centre of gravity away from the capital and transform connectivity between our towns and cities".
HS2 must be "delivered more efficiently and cost-effectively so that communities feel its benefits more quickly, particularly those in the North," he added.
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