By Tom Symonds BBC transport correspondent

Only half the population uses trains each year

How much does it cost to run a railway? A decent one, that is.

The man who had to answer that question is Tom Winsor, the rail regulator. His considered conclusion was: a lot more than that is being spent now.

Here are the numbers:

Mr Winsor has decided Network Rail's budget should rise from around �15bn over five years, to �22.2bn.

Network Rail itself wanted more than �24bn, and even that figure has been radically reduced from the �34bn the company first asked for.

'Constitutional crisis'

So who is going to have to come up with this extra �7bn?

The money is actually made up of increased charges which train companies pay to use the tracks. But under their contracts train companies are protected from footing the bill.

So they simply pass it on to the government - which has to find the money.

The government cannot simply say it will not pay up. If it did, the post of rail regulator would become untenable, creating a sort of "constitutional crisis" in the railways.

�22bn rail investment approved

In fact the Transport Secretary Alistair Darling recently confirmed the role of independent regulator was not up for negotiation.

However, almost in the same breath, he said it was important that the government alone decided how tax-payers money was spent.

As a result in the last month Network Rail and the government have been doing just that.

Tom Winsor's now agreed to let Network Rail borrow more than �3bn over the next two years - when it has to spend a lot on repairs - and pay it back later, when the rail system's in a better condition.

He is also allowing the government to give money directly to Network Rail rather than giving the train companies more money to pay higher charges for using the track.

It is a bit of financial sleight of hand which will make it easier for Chancellor Gordon Brown to balance his books.

More money

But either way he will still have to find the money.

As senior figures in the rail industry are fond of pointing out - there are only two sources of cash for the railways - fares from passengers, and tax-payers money from the government.

Mr Darling has ruled out raising fares beyond the small increases already announced.

So it does seem that when the government carries out its spending review next year, it will have to find more money for the railways.

Network Rail has a huge backlog of repairs

So is there a passenger benefit?

A difficult sell politically, given that half the country does not catch a train at all from one year to the next.

Even once this is resolved, passengers are being promised little more in terms of train services than they have had in the past.

Network Rail says only by the end of the decade will it be able to say fewer than one in ten trains are late.

British Rail was achieving that back in the 1990s.

At least there are many more trains services running than there have been in the past, and hundreds of trains are new.

Passengers can only hope that if these big decisions are taken now, at least some time in the future Britain's troubled train services will improve.