Train doors at the rear DT car were notproven closed.TC opened and closed train doors a few timesbut to no avail.Station staffassisted to isolate 'A1' d oor for train to depart. ~~~~~ L3 3540 A1 door checked for FOD - Not Found. Door operation test - OK. Push/pull gauge test within criteria. As preventive measure, replaced 3541 A1 door actuator & cleared the J-rail - Done. Door gauge (10mm/20mm) & push/pull gauge test - OK. Cyclic test 20 times - OK.  Selvan/Lee PT. 20/03/2020. L3 3540 A1 door manifold v/v replaced as instructed. Re-installed the previous door actuator that was removed. Door operation test & door gauge test - OK.  Chan GF/Alan/Alcus. 21/03/2020.