Railroads have been using disk brakes on passenger cars for more than 60 years, but coupled with a "Rolokron" anti-lock system to avoid the creation of "flat spots" (or “"square wheels"”) when wheels lock and skid on the rail surface (audible as steady "bang-bang-bang" noise as a train goes by—not to be confused with the bang-bang...bang-bang...bang-bang sound made by wheels rolling over a rail joint). Usually, brake disks are installed in the center of the axle, but in some applications (such as Bombardier Bi Level commuter cars), only one disk is used, mounted on the axle end outside the truck frame. High speed trains (such as the TGV) may use four disks per axle.