Tube drivers are set to reduce train speeds in protest at the levels of noise they are being forced to endure on the Victoria line.
The “action short of a strike” follows an overwhelming vote among RMT members in favour of an official protest about noise levels on the line, which has the most frequent and some of the fastest trains on the London Underground.
Fellow union Aslef, which represents about 2,000 Tube drivers, has similar concerns that extend to three of the other “deep level” lines – the Central, Northern and Jubilee.
It says that London Underground, after years of denials, has admitted that noise levels on the Victoria line “exceed legal limits”.
Aslef health and safety reps are meeting Tube management in a bid to identify solutions to the longstanding problem.
Unless changes are made to rapidly address the issue, Aslef members on the Victoria, Central, Northern and Jubilee “will be taking action”.
This is likely to involve balloting drivers on whether to go on strike or take action short of a strike.
RMT sources said that the action short of a strike would involve driving Victoria line trains below 50mph in a bid to reduce the noise that is caused by train wheels screeching against the rails or rumbling over sets of points.
The Victoria line is largely automated and the trains “drive themselves” between stations, though drivers can take control if required in an emergency.
However, drivers have full control of the train when it is being driven in and out of the depot.
Even a small reduction in the speed of one train could cause knock-on impacts along the line and make it impossible for the Victoria line to achieve the peak hour frequency of 36 trains an hour.
Transport for London has spent almost £30m trying to combat Tube noise in the last five years, including on “grinding” the rails to try to remove bumps and abrasions.
Passengers have complained about excessive noise – while residents living near stations have reported disturbance.
Concerns grew when the Night Tube - which operates through the night at weekends - launched in 2016.
Problematic sections include the Central line between Stratford and Leyton, much of which is tunnelled.
The RMT Victoria line ballot saw 102 of 159 members take part, with 100 voting yes to going on strike and two voting no.
In addition, 101 backed action short of a strike, with one saying no.
According to Aslef, Transport for London has been required since last month to offer all Victoria drivers “mandatory hearing protection” when they clock-on.
Last month Finn Brennan, Aslef’s district organiser, said Underground managers were “applying a band-aid approach to the symptoms”.
Writing in the Aslef newsletter, He said: “Issuing ear defenders cannot be an alternative to action to actually reduce the noise level.
“This is a problem that would have been avoided if management had acted earlier. But they choose to bury their heads in the sand and ignore the complaints of drivers and union reps.
“Everyone has the right to a safe working environment and to act to protect their own health and safety. Aslef will always support members who take appropriate steps to stay safe at work.”
TfL has been approached for comment.
