NORTHERN LINE trains introduced 10 years ago are causing signal failures and worse than expected damage to track, London Underground maintenance contractor Tube Lines said this week.
The company said the problems affected sections of track between 100m and 700m long at 17 locations.
Problems go back to 1995 when Northern Line track had to be lowered to accommodate a new fleet of trains.
'The alterations were done in a cheap and cheerful manner.
The life span [for the design] was only three to fi ve years and the follow-up permanent solutions were not carried out, ' said Tube Lines Northern Line infrastructure manager Lee Jones.
Jones added that the larger 1995-stock trains were heavier and applied greater acceleration and braking loads on rails.
'Wheel loadings on the rails are different to what they were and [grind away] metal shavings which fall on the track and affect signalling, ' added Jones.
Rails were lowered to accommodate the trains by using thinner base plates and rubber shoes between track and the tunnel invert.
'But the rubber shoe degenerates over time, so you start getting movement [in the track], ' said Jones. As base plates became more mobile, holdingdown bolts have loosened and gouged larger holes in the concrete tunnel floor.
In some cases the bolts have come into contact with reinforcement within tunnel inverts.
This in turn has short circuited signals powered by low voltage current running through the track.
Design of the trains is causing other problems, Tube Lines said.
The new fleet included a self steering bogie which automatically picks out the best path to take on curved sections of track.
'But that doesn't always suit the train or the track. The Northern Line has more rail defects than any other line on the London Underground, ' said Jones.
Repair work involves lifting the existing rail, removing the base plates, breaking out concrete, and pouring new concrete either for sleepers or a new base section. Base plates would then have to be set and levelled before replacing the rail (see diagram).
At critical locations the contractor has installed clamps as a temporary measure on either side of each rail to restrict lateral baseplate movement.
It wants London Underground to let it have long running track closures lasting several weeks so the problem can be cleared as quickly as possible.
Ruby Kitching l ondon Underground this week agreed to a five month closure of the Waterloo and City Line from 1 April 2006. This will enable contractor Metronet to upgrade track and renew electrical, mechanical, fire protection and communication equipment without interruption from daily train services. Rolling stock will also be refurbished.
The closure follows months of negotiations between LUL, Metronet and local businesses.