Train had slow powering departing YWT1,could only achieved 7Kph with no fault indication. TC operat ed STBS and proceeded 35kph to CCK1 and disembark passengers . ~~~~~ L3 507/508 checked BRER line & found 3507 ABPR relay not energised. Swopped BCU to 1507 & fault transfer. Replaced BCU at 1507 - Done. Function test & limited movement - OK. 1507 BCU S/N : OLD : 1745008. NEW : 1745005.  Chan GF/Alcus/Alan. 19/03/2020.