Chris Grayling has been accused of hiding information on a government decision to scrap rail electrification plans outside London.
The transport secretary was quizzed by MPs on the transport select committee on Monday over why his department had decided to scale back schemes which would have provided faster trains between London and cities in South Wales, the Midlands and Yorkshire.
Committee chair Lillian Greenwood, a Labour MP in Nottingham, asked why she had not been provided with proper evidence of why the government decided to replace electrification schemes with upgraded bi-mode trains - which can be powered by either diesel or electricity - despite repeated requests.
Grayling insisted bi-modes offer value for money - but experts say they are more expensive to maintain.
“My job is to try to maximise the value to passengers of the investments that we make,” he said.
Greenwood told HuffPost UK passengers who had missed out as a result of the schemes being scrapped would “rightly remain angry and let down”.
She added: “The evidence we heard raises plenty of further questions. The secretary of state’s argument was that the use of bi-mode trains is a better use of money, delivering almost identical benefits to passengers and that with a finite amount of money, cash can be spent to address capacity constraints elsewhere.
“However, we still don’t feel confident that the secretary of state has justified his decision.
“It is vital that the department is more transparent than it has been so far.
“If we take the justification given by the secretary of state at face value, then it shows some frankly astonishing decisions made by his predecessor.”
She said the committee would “reflect” on Grayling’s evidence before deciding what to do next.
During the session, the minister also admitted taxpayers would lose out as a result of Virgin and Stagecoach handing back the East Coast Mainline franchise earlier than planned.
The firms promised returns of about £3 billion during the eight-year-deal struck in 2015, but passenger numbers are lower than expected and Grayling said “lessons had been learned” by his department as a result.
Shadow transport secretary Andy McDonald said: “It’s hugely disappointing that Chris Grayling still refuses to publish the evidence and explain why he’s cutting rail infrastructure upgrades and letting private companies off the hook.
“If he has nothing to hide he would share information and give direct answers to questions about electrification and the East Coast contract.
“Taxpayers and passengers deserve a straight answer from the secretary of state.”