New trains being introduced in Sussex are too heavy for the tracks.

Safety experts have warned that the class 377 Electrostars, which weigh ten per cent more than the slam-door stock they are replacing, will wear the rails down more quickly.

Network Rail, responsible for track maintenance, expects the problem - known as rolling contact fatigue - to worsen in the next decade as the rest of the £850 million fleet of 700 new carriages is brought into service.

Rolling contact fatigue is the first stage in the flaking of steel rails.

If not checked early on it can lead to rails disintegrating.

The phenomenon was blamed for the Hatfield rail disaster in October 2000 in which four people were killed.

Hatfield led to a major nationwide track renewal and maintenance programme and speed restrictions.

More frequent tests and repairs will lead to more diversions and delays.

Concerns about track wear are revealed in Network Rail's 2004 business plan.

It says: "The impact of the introduction of class 377 rolling stock on track renewal and maintenance requirements is being assessed, taking into account the higher tonnage of the new trains.

"It is anticipated that the suspension characteristics of new trains will lead to a significant growth in rolling contact fatigue propagation with consequential impact on track maintenance activity."

Weight is the latest problem surrounding the new carriages' introduction in an £850 million rail investment inherited by the main rail company in Sussex, which now calls itself Southern.

The county's first new train broke down on its first day.

Donna Casey, spokeswoman for Network Rail in Sussex, said the tracks had been continuously monitored for stress since the fleet was introduced and so far no increase in wear and tear had been detected.

She said: "We have mentioned rolling contact fatigue in the ten-year plan as a contingency should we need to ask for more funding to cover this."

Southern spokesman Marsid Greenidge said: "Network Rail has confirmed with Southern that at this stage of the introduction of our new trains there has been no detrimental impact on their track maintenance programme."

Rail commuters were concerned. Henry Law, 63, a design consultant who lives close to Brighton station, said: "It is one thing after another with the new Electrostars.

"I far prefer the slam-door trains - and I'm not alone."