Line tests of wheel-rail adhesion recovery under braking using an electric multiple unit train

Rail surface contaminations causing wheel-rail low adhesion problems may gradually be cleaned by passing wheelsets, leading to the phenomenon of adhesion recovery. This suggests an enhancement in available adhesion under low adhesion conditions for a train composed of multiple coaches. Using a four-coach Electric Multiple Unit (EMU) metro train equipped with a wheel slide protection system, field tests are conducted on a test line to measure the adhesion recovery rate of braking wheelsets under soapy water contamination. Initial braking speeds range from 100 km/h to 140 km/h, and tests under dry conditions are also conducted for reference and braking force calibration. Instantaneous creepages for different wheelsets are determined by recorded train speeds and rotational speeds of wheelsets, and the corresponding creep forces are calculated by measured vehicle speed, angular speeds of wheelsets, and brake cylinder pressures. Creep curves for different wheelsets are then obtained in consideration of the calculated axle load transfer. Adhesion coefficients corresponding to friction saturation are further derived for sliding wheelsets, typically located in the leading part of the train, with a maximum measured creepage of up to 11%. The number of passing wheelsets required to achieve adhesion recovery meeting braking requirements is also identified. The results indicate that the adhesion coefficient is as low as approximately 0.075 of the 1st axle at speeds between 93 and 132 km/h, with an adhesion recovery rate of 0.0040/axle among the first seven wheelsets. Such measurement technology holds potential application in operational trains, as it requires no significant modifications.