Errors made in London's £7 billion Thameslink rail programme must not be repeated when HS2 is built, MPs have said.
The Department for Transport (DfT) and Network Rail should establish "formal processes" to ensure lessons are learned, the Public Accounts Committee (PAC) recommended.
In October the DfT decided to delay the full introduction of new Thameslink services running north-south through the capital from December 2018 to December 2019.
This reduces the risk of disruption but means passengers in some locations will not receive the full benefits of the programme for up to a year later than scheduled.
The committee found that it took the DfT and Network Rail too long to start planning how the new railway will operate.
It recommended that both bodies should "establish clear arrangements at the outset of future programmes to plan how services will be introduced and run".
PAC chairwoman Meg Hillier said: "Passengers and the practicalities of running services should be at the heart of public transport planning.
"On Thameslink these considerations came too late and Government faced a stark choice: delay the rollout of services or risk additional disruption on the network. Either way, passengers lose out.
"Government must apply what it has learned here to HS2 and future programmes and in response to our report we urge it to demonstrate how it will do this in practice."
Thameslink services will step up to 18 trains per hour in May and 20 per hour in December, with the full increase to 24 not reached until December 2019.
It is not the first time an increase in services on the route has been delayed.
A modernisation scheme named Thameslink 2000 was first discussed in 1991 but did not begin until 2006 due to privatisation of the railways and complex planning inquiries.
A DfT spokesman said that expanding the Thameslink network will "transform travel across London and the South East" with new trains providing faster, more frequent and more reliable journeys.
He added that the most recent independent review of HS2 Ltd concluded that it is "ready and fully capable" of effectively delivering contracts to build the high speed railway.