High-speed rail services on HS2 are set to operate at slower speeds than originally planned, in a move aimed at curbing the project’s spiralling costs.Transport secretary Heidi Alexander is expected to confirm to the Commons on Tuesday that the maximum speed for trains will be capped at 320kmh (199mph), a reduction from the initial design of 360kmh (224mph).Despite this downgrade, the services will still outpace Japan’s renowned bullet trains, which typically reach speeds of up to 285kmh (177mph).HS2 services will still be faster than Japan’s bullet trains (PA)Trains run at speeds of up to 300 kmh (186 mph) on HS1, which travels through Kent and internationally via the Eurostar.A major review to be published following the announcement is expected to find that “gold plating” HS2, including by focusing on achieving the “highest possible speeds”, is among the faults that contributed to the project’s difficulties.Sir Stephen Lovegrove, former national security adviser, is poised to criticise the "original sins" of the scheme's decision-making.Ms Alexander is also expected to unveil a new target cost for the project, which will be set below £100 billion. The initial estimate for constructing the line from London to Birmingham, alongside the now-abandoned onward legs to Leeds and Manchester, stood at £32.7 billion in 2011 prices.However, the budget has ballooned significantly, with the estimated cost for the London-Birmingham section alone reaching £66.6 billion by January 2024.Transport secretary Heidi Alexander will give an update on HS2 (PA)The transport secretary will further outline a revised schedule for HS2’s opening. It has already been confirmed that the previous target window of 2029 to 2033 cannot be met, a significant delay from the original plan for the first phase to launch in 2026.A government source commented: "The Lovegrove Report further confirms the astonishing extent to which previous Conservative governments had totally lost control of HS2, frittering billions of taxpayers’ money away and leaving the project no closer to being finished than when it started.“It has been a sorry mess, but this government has done the hard yards to pull the project out of the dirt and deliver the better connections that have long been promised to the Midlands.“Britain has the talent and capability to build big infrastructure projects. The transport secretary will harness that as she turns the project around.”