Failings in the Great Western rail line upgrade raise concerns over similar projects, an influential parliamentary committee has said.
The Department for Transport (DfT) and Network Rail must learn from significant flaws in the design, planning and cost estimates of electrifying the line between London and South Wales, according to the Public Accounts Committee (PAC).
The problems cast doubt over planned electrification schemes, the MPs warned.
The scheme's budget rose from £874 million in 2013 to £2.8 billion in 2015.
Ms Hillier said "the sums of public money wasted are appalling" and accused the DfT of failing to adequately challenge Network Rail's plans.
Some sections will not be electrified until 2019-2024, despite the project being due for completion next year.
Trains powered by electricity rather than diesel can have more seats for the same length and be faster, quieter and result in fewer carbon emissions.
The DfT has claimed that many of the passenger benefits of electrification can be obtained without electrifying the whole Great Western route.
This raises questions about whether full electrification is the most appropriate action, the PAC found.
The department should reassess the case for electrification on a section by section basis and only support schemes where the benefits could not be achieved at a lower cost, according to the report.
Mark Carne, chief executive of Network Rail, said the Great Western scheme was agreed in 2009, "long before the scale of the work was properly understood".
Network Rail and the DfT have learnt the lessons from the poor early planning, he insisted.