Safety engineers have been asked to check if an £850 million fleet of new trains is damaging tracks.

There are fears the new Electrostar trains could be putting too much pressure on the line and moving rails out of alignment as they go round bends.

Wheel-assembly bogies on 700 carriages which will eventually make up the entire stock operated by Southern in Sussex do not turn as freely as those on the slam-door trains they are replacing.

This puts more side wear on the track, especially on routes where there are a number of curves, such as between Brighton and Seaford.

The wheels on the new trains are also smaller than the stock they are replacing, again exerting more pressure.

Experts believe this will lead to more track maintenance and track replacement programmes.

Some say tracks could move out of line unless the problem is solved.

The new fleet has been beset by several troubles since the Electrostars were introduced.

Problems included toilets locking when their waste tanks became full.

This led to drivers making longer stops at stations to allow desperate passengers to use station toilets.

Engineers say there could be problems with the track because of the rotational stiffness of the bogies, designed in to stop excess movement.

Side wear adds to contact fatigue by the heavier Electrostar trains.

Roger Ford, industry and technology editor of the magazine Modern Railways, said: "The Electrostars go round the curves all right but they exert more pressure and this will lead to greater side wear,and more maintenance and track repair programmes."

Southern has inherited the Electrostar programme which will replace its entire slam-door stock with trains made by Bombardier at its Derby works.

The rail company has been ordered to get rid of all its ageing slam door trains by November 2005.

There has been a history of problems since the Electrostar trains were first introduced in December 2002 - the first broke down on the first day in service.

An emergency programme to boost the power supply had to be introduced because Electrostars use more current.

There were problems with doors shutting too quickly and also not quickly enough.

Passengers roasted last summer because air conditioning did not work properly Henry Law, a retired designer, who was involved in an assessment project for the old slam-door trains, said: "The problem with the new trains is they are being introduced too quickly.

"They should have introduced them, found out the problems, and them made changes to the design, instead of sticking to one design for the entire fleet.

"If it is found the faults are the result of bad planning and design I believe it is the manufacturer, not the taxpayer, who should pay."

Donna Casey, spokeswoman for Network Rail in Sussex, said: "We are closely monitoring the tracks for additional side wear as a result of the new trains. There has been no significant change to date on the Sussex routes.

"Should changes be found, Network Rail will carry out maintenance work to deliver a safe and reliable railway."

Southern spokesman Marsid Greenidge said: "We are confident we will be made immediately aware of any changes to track conditions caused by the new trains."