When previous generations embarked on visionary transport projects, private companies clubbed together to raise money from their shareholders, and made them happen.

The Forth Bridge for example, completed in 1890 at a cost of £3 million (£1.3 billion in today's money), was built by four rail companies without parliamentary committees, political footballing, consultants, media campaigns or social impact assessments.

Nostalgia for Victorian values in construction has its limits; 63 workers died building the Forth Bridge. Nevertheless, as is clear from the delay surrounding major UK projects, from High Speed 2 (HS2), to the third Heathrow runway, to the Edinburgh trams to the Borders Railway, advances in the delivery of infrastructure are less obvious than progress in health and safety.

When it comes to big transport projects, political priorities are impeding the UK's ability to compete internationally and to shore up the country's shaky finances.

Consider HS2, the projected fast north-south rail link intended to "bring the UK's 19th-century railway infrastructure dramatically into the 21st century", and already a political football in what looks set to be a long, exhausting match.

At a cost of £50bn and with a timeframe of more than 20 years, a project of this scale requires cross-party support, and had it until last month, when shadow chancellor Ed Balls, realising that support for HS2 was receding, pricked the consensus by hinting that Labour might not back it.

And so a painstakingly constructed cross-party case for HS2's contribution to regional equity was swept aside by a politician with an eye on the headlines.

In Scotland, "exhibit A" in the case for taking the politics out of what should be economic decisions is the Borders Railway, where politics have trumped economics since conception. First proposed 10 years ago, construction of the Borders Railway started six months ago, and is proceeding successfully, according to Network Rail's project website. It has strong support in Midlothian and the Borders, which have missed out on big transport projects in the past.

But to critics like economist Tony Mackay it is "the next Scottish transport fiasco after the trams" and one that shames the civil servants he believes reverse-engineered the economic case to please the scheme's political sponsors.

To the former Tory MSP Brian Monteith, the Borders Railway is "Holyrood's greatest folly to date" and its carriages, at least in the southern, more rural portion of the 30-mile track, will be largely empty.

The pattern of the last 10 years has made it easy for critics to present a single-track, diesel railway serving a low-population area as a potential white elephant. Initially presented as a means of stimulating construction en route, the line has been repackaged around the political priorities of "accessibility and social inclusion".

Costs have spiralled from an initial £73m to £350m and years have been wasted in an attempt to find a new way of financing the route to lessen its impact on a Scottish budget already stretched by other ministerial promises. The completion date has receded from 2011 to 2015. Despite rumours to the contrary, this is a date that builder Network Rail is confident of meeting.

The underlying criticism of the Borders Railway is that it is a politically inspired construction of the previous Scottish Executive, reversing a notorious Beeching cut in a Liberal Democrat-voting territory.

Whatever the politics, the final business case (FBC) makes clear, implicitly and explicitly, this project has bust the boundaries of the Scottish Transport Appraisal Guidance (STAG), the technocratic methodology devised to prioritise transport schemes. Written by accountants and consultants Ernst & Young (E&Y), the business case document was completed last November but withheld for more than three months, a delay explained by the need "to ensure any information contained within a publicly available FBC does not prejudice substantially the commercial interests of Scottish ministers in a live contract environment".

The reasons for such lengthy consideration are not obvious. When Transport Scotland did publish, it simply redacted the entire commercial and financial case for the railway. The most significant claim in what is publicly available of the report is the admission that the benefit cost ratio (BCR) - the measurement of bang for the buck - had "significantly worsened" from the free-spending days. In economic terms, the project has a negative BCR of 0.5, which means every pound spent yields only 50p in return. The net present value of the project is negative £63.3m, which can be adjusted to negative £37.5m if "wider economic benefits are included".

According to Tony Mackay, E&Y's report contains "no explanation as to why Transport Scotland wish to proceed with a project with a negative net present value (NPV)".

It is unusual to proceed with a project that has a negative NPV and BCR. E&Y show that by including "accessibility and social inclusion benefits" it is possible to present the BCR as 1.3.

The careful language deployed to explain the use of these and other "wider economic benefits" to reach a positive figure suggests a degree of professional circumspection on the part E&Y.

"The economic benefits ... have been calculated through the use of a bespoke [specially constructed] model, which is based on standard rail industry modelling techniques and is in line with [Scottish guidelines]. The methodology for calculating these additional impacts is an emerging area of transport economics, and is generally accepted as being less certain than the calculation of standard benefits."

Professor Tom Rye, formerly of Napier University and now director of the Swedish National Public Transport Research Centre K2 in Lund, Sweden, coments: "You don't have to be a genius to see the business case for a line that goes to a place where not many people live doesn't stack up, and that the people who commissioned the E&Y report and still decided to invest in this project are being more than disingenuous. The responsibility rests squarely with the civil servants at Transport Scotland, not with E&Y.''

Transport Scotland's statements about the railway suggest the decision to build it was a political one back-filled with hope, assertion and downplayed risks.

The search for a justification for the Borders project has also entered the more exotic realms of what are called "option and non-use values", essentially the value of knowing the railway is there, even if you don't intend to travel on it yourself. This figure is valued at £130m. The agency has called in the aid of a calculation by the Department for Transport of "wider economic benefits", based on mind- bending equations of effective density, GDP per worker, and employment.

Says Transport Scotland: "These benefits are dominated by the monetised valuation of journey time benefits for existing and new journeys, but also comprise decongestion benefits through modal shift to rail. Environmental and safety benefits have also been estimated at an additional £2m and £5.8m respectively."

"Bull***t," claims John Carson, a former director of maintenance for Network Rail. He says: "Any 'modal shift' is included in the projected 647,000 passengers who won't be driving their cars, included in the dire original BCR, so you can't add an extra category that counts them again. Running diesel trains even Transport Scotland predicts will be virtually empty south of Gorebridge cannot be counted as positive in environmental terms."

It seems unlikely anyone will ever be held accountable for the politically helpful claim the Borders Railway can be weaned off public subsidy.

Among those who think projects like HS2 and the Borders Railway are a poor use of public funds are those who believe the only way to prevent the politically expedient over-optimism of transport projects is to threaten legal sanctions in the case of claims that turn out to be erroneous.

Such a harsh approach might seem more in line with the 19th century than the 21st century.

Nevertheless, the public might have confidence that big projects such as the Borders Railway or HS2 were going ahead because, like the Forth Bridge, they had satisfied rigorous business as well as social benefit criteria, independent of political whim, and could therefore go ahead as quickly and cost-effectively as possible.