While the politicians continue their debate on how best to fund improvements to the Tube, JULIAN HILLS investigates the size of the task that lies ahead:

Battling their way onto a smelly, dusty, delayed Northern Line Tube every morning, most commuters in Barnet could be forgiven for feeling ever-so-slightly frustrated at the lack of progress made in negotiations to bring improvements to the London Underground.

After months of talks, Transport Secretary Stephen Byers announced last week that the Government's plans for the part-privatisation of the Tube would go ahead, whether London Mayor Ken Livingstone likes it or not.

"The Tube must be publicly owned, and publicly run," Mr Byers proclaimed, in an attempt to assure passengers that there would not be a 'Hatfield'-type disaster on the Tube.

"There will be no privatisation of the Tube. London Underground must own and run the Tube. It will be publicly run and privately built."

The thrust of the Government's plan is to bring in £13billion of private cash.

But its argument did not convince Bob Kiley, appointed by Mr Livingstone to run the Tube, who is now due to meet the Government in court on July 23.

He thinks a safe and efficient Underground system cannot be delivered under the current public-private partnership (PPP) terms.

While all this goes on, Northern Line passengers continue to pay some of the highest fares in the world to travel on what has been described as one of the most overcrowded and dirtiest lines on the network.

Following a survey into dust particles on the Underground, health and environment expert Professor Nick Priest at Middlesex University, identified the Northern Line as the 'dirtiest' route, along with the Piccadilly and Central lines.

Dust levels are up to 73 times the recommended limit for air pollution above ground.

The Government first moved into attracting private investment into the Underground in 1997, when Golders Green-based GEC Alsthom supplied Northern Line passengers with 106 gleaming new trains, replacing some of the oldest carriages on the network.

But while the old trains may have disappeared, ancient signalling remains, leading to regular breakdowns, speed restrictions and delays.

The Tube Lines Group (TLG), a consortium made up of three shareholders Amey, Bechtel and Jarvis, is the preferred bidder for the Northern, Jubilee and Piccadily lines, and is expected to take up its role in the PPP in November this year.

TLG spokesman Chris Murfitt said the Northern Line signals have been in operation since 1936 and are the oldest on the Underground network. In fact the signals are the main reason behind the unreliability of the service.

"We are going to start replacing the whole signalling network and that is going to take some time, and we are going to do it without shutting down the service," he said. The whole operation is expected to take seven years with the aim of increasing the service from 27 trains an hour at present to 33 an hour.

Hendon Labour MP Andrew Dismore, acting as one of the executive group of London Labour MPs, was one of the outspoken critics of his own Government's decision to push through the PPP last week, arguing negotiations between Kiley and the Government must continue to address lingering safety worries.

"In principle, there is no reason why private sector involvement cannot work," he said.

"My main concern is not even about money. My main concern is, is it workable from a safety point of view? Once you get a convoluted chain of command, then that is a recipe for a breakdown in communication."

Mr Murfitt argued there was a clear line of communication between the public and private sector.

"The Government has made it clear that if Transport for London or London Underground have any concerns about anything, they will be able to step in without notice or without compensation and order us off," he said.

The Government plans for the PPP, including modernisation programmes for Brent Cross, Burnt Oak, Edgware and Golders Green stations, were attacked by Mr Dismore as misguided.

"What are people's priorities?" he asked. "Tarting up the stations or making the trains run? The private sector can make more money out of tarting up the stations more quickly, as they can build shops and offices on the stations. Kiley's priority is to get the trains to run."

So, aside from all the complex arguments about chains of command, safety concerns and private control, the issue boils down to getting a sufficient number of trains to run smoothly and on time.

Whether it is the private sector or the public sector that wins out, the huge backlog of under-investment in the Tube means that it will be years before passengers get the service they deserve.