The trains will reduce journey times between London and Scotland
The high-speed tilting train project on the West Coast Main Line has been hit by more problems after tests revealed it can interfere with signals.
The hitch was discovered during a non-passenger test run of the Virgin Trains Pendolino train between Crewe and Liverpool.
It was discovered that electromagnetic interference from controls driving the motors on the trains
can change the lights on the signals.
The roll-out of the service, which is planned to run between London and Scotland, has already been subjects to delays.
Network Rail - the company that has taken over from Railtrack - said it was now discussing the problem with Virgin, the Strategic Rail Authority and the
Alstom company, which is building the Pendolinos.
There is speculation that trains' traction motors might have to be re-designed and that special filters will need to be fitted to the signals.
But a Virgin spokesman insisted on Friday that the company did not anticipate having to put back the autumn 2004 date for the Pendolinos to switch from 110mph
to a full tilting mode of 125mph.
The trains were due to be introduced in full 125mph tilt mode on the West Coast line in May 2002.
But a series of delays have seen the cost of the West Coast upgrade reach £9.8bn and have meant the Pendolino project timetable has slipped.
Virgin has so far received 15 of its 53 Pendolinos.
But they are only running at 110mph in non-tilt mode and only on Tuesdays between London and Wolverhampton and on Wednesdays and Thursdays between London and Manchester.
Virgin hopes to run Pendolinos on five days a week by the end of the summer and, by 2006, reduce journey times between London and Scotland by about an hour
to four hours 33 minutes.