The bow wave effect occurs when the train speed approaches the velocity of surface waves generated by the train in the foundations, typically at speeds of around 150mph - an effect similar to the sonic boom created by supersonic jets.
At low train speeds the surface waves travelling along the track have time to dissipate, but at high speeds the train catches up with these waves, and adds more on top. The waves interact causing vibrations as great as 20mm. These vibrations shake the ballast, which can lead to track alignment and sleeper support problems, requiring frequent remedial work.
Problems with the bow wave effect have caused speed restrictions to be imposed on high-speed lines at Ledsgard in Sweden and at Stilton Fen near Peterborough.