Rear DT B3 door not proven closed. Openand close train doors once. Fault cleared. Fault recurred atWDL1. Staff(WDL) on board check need to push close at MSL1. Staff(WDL) isloate in closed position at KRJ1. ~~~~~ Rear DT B3 door not proven closed. Open     #3703 Downloaded ER and fault traceon PTU 006.Shown FC: 973.From door PTU,observed when #3704-B3 door is closing,operation is not smooth.Checked door guide rail and J-rail for FOD,found J-rail with large dumps of dirt.Cleared and serviced J-rail and door rollers.Carried out door push pull gauge test-4kg force ok.Door mating strip 64mm ok.Carried out door cyclic test ok. Done by Jack/Sateesh/Koh. 07/04/20