The high-speed Javelin trains operating between Kent and London are having to be modified, it has emerged.
The Southeastern train company said the Japanese-built trains had experienced "a slight sideways movement" and correction work was being done.
The company said work had so far been carried out on 10 of the 29-strong fleet of trains.
The trains run on sections of High Speed one (HS1) - the London to Folkestone Channel Tunnel rail link.
The 140mph trains will be used to take spectators from St Pancras station to the 2012 London Olympic site at Stratford in east London in just seven minutes.
A Southeastern spokesman said: "We have modified 10 trains so far and we will complete the work on all 29 trains in the next couple of months."