TC feedback train unable to pick up speed in Auto and ATPM mode. No other fault indication. Train di sembarked pax at BNL EB and later operated STBS to proceed b ack to depot in ATPM mode. TC feedback train unable to pick ~~~~~ L3 523/524 ER showed BRER intermittently unable to energise during time of fault. No FC found. When in depot, found #3523 BRAKE O  Auxiliary relay (BO AR) unable to de-energise when MCH put to COAST, resulting BRER unable to energise to allow propulsion to kick in. Checked 6 cars ABRR1 & PBPR, M-cars ELB2R coils & contacts - satis. Checked ER history of brakes applied/released - satis. In depot, brake cyclic test, BCP within criteria. All brakes able to physically release when at COAST. Swapped BCU from #1523 to #3523.    Replaced new BCU spare at #1523. Function check brake cycle B3 to COAST - both satis. BRER able to energise post #3523 BCU c/o. BCU S/N : #3523 - 1809061 (OLD)  1815132 (NEW). #1532 - 1815132 (OLD)  1820027 (NEW).  Lee JC. 10/03/2023. Depot investigated and found 3523 BCU faulty, causing BRER unable to energise. Depot replaced faulty BCU. Train released on 10/03/2023.