Professor of economic history at the London School of Economics, Dr Tim Leunig, says Conservative proposals to build new high speed rail lines are based on 'funny money', and the number do not add up.
Speaking to NCE at Civils 2008, Professor Leunig said high speed rail was: "very expensive. It is notable that the Conservative have only funny money [to fund it], and will use the existing rail budget.
"I think we should only take politicians' spending proposals seriously when they actually have some money on which they can write the cheque."
Professor Leunig said the idea makes sense only when you have large numbers of people travelling on a particular line, and that lines from London to Manchester were 'conceivable', but large numbers of lines were not.
Greengauge21 founder Jim Steer said he believed two corridors had 'very strong' demand now - the West Coast and East Coast Mail Line corridors, but beyond Manchester and up to Scotland.
"They are claiming that they can find £1bn in savings from the existing rail budget, and they have been pretty vague about where that extra £1bn is going to come from.
Professor Leunig said the Conservative plans should be treated with scepticism until there was more detail.
Steer said: "The costs to run Network Rail are roundly £5bn a year. Adding a further £1bn of investment, and I would like to see it as additional investment ... doesn't seem to me an implausible way to allocate our scarce finances to investing in infrastructure."
New high speed lines would gobble 25% of the capital rail budget between 2015 and 2017, at a cost of £1.3bn per year between 2015 and 2027.