Deputy Prime Minister John Prescott's promise that "cost will not be a consideration" in implementing the "best possible" safety system in trains marks a stark departure from the calculations usually performed in such deliberations.

The traditional and accepted approach to considering the practicality of installing new systems involves weighing their cost against the value of lives that they may save.

And that, of course, necessitates coming up with a figure for the value of a human life.

Andrew Evans, Professor of Transport Safety at University College London, says that after a tragedy has occurred, the value of a human life is "priceless".

Loss of human life

However, in anticipating the risk of loss of life, whether on the rail network or on the roads, a value is calculated and applied to cost effectiveness.

Prof Evans said: "The hypothetical cost of a possible human fatality is costed and given a value, and that value is then used as an indicator in calculating the cost effectiveness of installing a safety system, for example.

"In running a railway network, or a road system, you know that there is some risk, and you know that that risk is going to result in the loss of a human life.

"The crucial thing is though that you are dealing with an abstract - the death could involve anyone."

Deputy Prime Minister John Prescott: "Cost will not be a consideration"

A number of different factors affect the "value" given to a hypothetical human life, which differs from one situation to the next.

Although many, many more people die and are injured on the roads every year than in train accidents, a road fatality is costed out somewhere in the region of �1m.

The possible death of a railway worker is valued at about the same, or a fatality of someone dying as the result of vandalism or, for example, falling from a platform or bridge.

The potential death of a rail passenger, however, is priced at �3m.

The high profile given to rail accidents is one large reason for the increased value, Professor Evans told BBC News Online.

That value is then inserted into an equation to calculate whether the cost of implementing a new system is worth the number of lives it could possibly save.

"Not an easy decision"

In very blunt terms, cost benefit analysis has already shown the Automatic Train Protection system to cost more than the lives it might potentially save would be worth.

The Hidden report following the Clapham rail crash, in which 35 people lost their lives, recommended that ATP be installed across the rail network.

But it was calculated that although a full ATP system would save lives, it would be at a cost of �14m per life saved.

Prof Evans said: "It is not an easy decision, and it is not one made coldly or heartlessly. The decision of whether to implement a safety system or not is extremely hard."

But he said that even after the west London crash, the decision not to install ATP was correct.

He said: "After the trials of ATP in 1994 the British Railways Board reluctantly concluded that it should be rejected for network-wide fitting because its costs were much too high in relation to the number of accidents and casulaties it was expected to save.

Non-costed safety measures were introduced after the fire at Kings Cross

"That decision was correct, but a difficult one to make."

He said that the number of lives ATP could be expected to save should be increased in the light of Tuesday's accident, but he stressed that he did not believe that the adjustment would make the system cost effective enough.

However, he backed the deputy prime minister's previous decision to install the much cheaper Train Protection and Warning System before 2004.

He said: "You get 70% of the benefits for 15% of the costs."

King's Cross

He added that costs had to be paid for somewhere, and that increasing the price of train tickets may force people to adopt less safe forms of transport still - like cars.

But if John Prescott does decide to defy economics and go with ATP, it would not be the first time that safety has come before accountancy.

Non-costed measures were put in place after the fire at King's Cross underground station in 1987.

And the growing public outcry may influence any decision - political or economic - to install the best safety equipment possible.