Final decisions on whether to build new high speed rail lines would be made following a full public debate, he said.
"I personally believe there is a strong case [for high speed rail], but I do not want to be forced into a decision," said Harris.
Network Rail has agreed to examine new rail routes, but has not reached a decision on whether those routes should be conventional or high-speed (NCE 26 June).
Harris had previously questioned the green credentials of high speed rail in a letter to Liberal Democrat MEP Chris Davies.
Harris claimed that the main challenge for the UK’s transport network was congestion and reliability rather than journey times.
"At the moment Network Rail is looking at new lines, and if you read the Eddington Report [on transport for former Prime Minister Tony Blair], he said there is an unproven case [for building high speed rail] for reduced journey times," he said.
"The environmental case for high speed rail has not been made – the benefits are not black and white. By 2020, capacity will be the most pressing issue."
Harris acknowledged that the high speed lobby had built-up a significant head of steam, with the Conservatives pledging to take high speed rail to the electorate (NCE 26 June).
"I want to make the right decision, not just because of political momentum," said Harris.
"Whether we build conventional or high speed rail – what is the problem we are trying to solve?
"I want to be sure that high speed rail is not a solution looking for a problem."