SIGNAL FAULTS which cause 'ghost trains' to register on London Underground's Jubilee Line stopped contractors meeting their performance targets last year, it was claimed this week.
Contractor Tube Lines said that rushed work to complete the Jubilee Line Extension (JLE) on time in 1999 left tunnel reinforcement connected to the signalling system.
This caused shorts in signalling circuits which in turn disrupted services as operators struggled to locate trains.
Tube Lines this week blamed these and other inherited problems for its failure to meet availability targets on the Jubilee, Piccadilly and Northern lines last year.
First year performance results released last week showed that delays due to infrastructure failures were down 30% but this did not meet the contractor's performance target.
'It's fair to say that we would have met the targets if it hadn't been for signalling problems at Green Park on the Jubilee Line, the Camden Town derailment on the Northern Line and overheating axle boxes on the Piccadilly Line, ' said Tube Lines operations director Stephen Peat.
Tube Lines engineers have only recently resolved longstanding Jubilee Line signalling problems which resulted in signals detecting non-existent 'ghost trains' at Green Park station.
The cause was traced back to track circuits installed by London Underground as part of the JLE project.
The signalling system is designed to register the presence of trains when they reach a point where a current can pass between parallel tracks via train axles and wheels.
But in the rush to complete the project by 31 December 1999 bolts securing the tracks to the concrete track bed were accidentally drilled into reinforcement in the concrete tunnel lining. This permanently completed the circuit.
'It's only a 6V circuit and for much of the time the circuit was blinking at about 5.2V even without a train passing, ' said Peat.
The system was designed to be fail safe, so if a voltage was recorded, no other train could pass along the track leading to trains coming to a standstill.
'Last October 200m of track were taken up four times before we found out what the problem was, ' said Peat.
Insulating pads have now been installed between bolt pockets and reinforcement to break the connection. Signalling problems have ceased in the area.
On the Piccadilly line, overheating axle boxes also caused delays. Heat sensors have now been installed to monitor the axle boxes until they are replaced at a cost of £20M by 2009.
The derailment on a Northern Line crossover near Camden Town also disrupted services.