Rail authorities believe they have finally cracked an embarrassing, expensive problem, which the Government has described as "a massive clanger".

Hundreds of carriages from train operator Southern's £856 million Class 377 Electrostar fleet have remained unused in sidings because the power supply serving rail lines in the South-East is too low.

The new carriages, which have air conditioning, sliding doors and computerised controls, use more power than was available.

Six months ago, Southern had only 172 of 742 carriages on the tracks.

Now the firm has almost broken the halfway barrier, with 344 new carriages in service.

Network Rail has announced it has completed the crucial part of its £750 million power upgrade project, with 12 "super-critical" supply sites upgraded.

The full replacement of slam-door carriages with the new Electrostars is still not expected to be completed until next summer.

The Health and Safety Executive has agreed to let Southern, South West Trains and South Eastern keep using the slam-door coaches until November 2005 if necessary.

Progress so far has been cautiously welcomed by passenger watchdogs.

Forty-nine new Electrostar and Desiro carriages have been introduced by the three firms in the past week, bringing the total used in the Southern region to 1,044.

Network Rail spokesman Steve Turner said: "Power supply is no longer holding back the introduction of new carriages into service."

Work is continuing on new electrical stations at Hove, Brighton and Littlehampton.

Shelley Atlas, of pressure group Brighton Line Commuters, said: "The sooner all the new carriages are running, the better, because we're not happy with the slam-door trains.

"The sliding doors are popular with passengers. They are safer than the slam-doors and mean trains arrive and get away on time because people can't hold them open.

"I suppose the timetable we have now is as good as can be expected."

Strategic Rail Authority chairman Richard Bowker said: "While there is still much work ahead, I am confident passengers in the southern region will experience a huge improvement in their daily journeys in the coming months."

Southern managing director Charles Horton said: "This is an important milestone for the new trains programme across the Southern region.

"We are confident we will deliver the remainder of the new trains over the coming year and ultimately provide a better journey for all of our passengers."

The problem dates back to the late Nineties when rail operator Connex, who held the franchise before Southern, ordered the new carriages without realising how much electricity they needed.

Last October, transport minister Kim Howells said: "Somebody dropped a massive clanger. It is one of the most inexplicable inheritances we have faced as a Government."

The 1999 Railway Safety Regulations demand all Mark 1 slam-door trains are removed by the end of this year.

But the HSE has said it is "minded" to allow the three firms an extension until the end of November 2005, with a final decision expected this autumn.