 In the major MRT disruption of 15 December 2011 , one C151 train ( trainset 067 / 068 , <unk> in the COI ) was stalled in the tunnel and its backup battery failed . The passenger compartments in <unk> experienced a blackout and loss of ventilation , leading to one passenger smashing a train door window to avoid suffocation . The battery failure led to criticism by the COI on the condition of the aging emergency batteries installed on all C151 trains . SMRT has since proposed an upgrading plan to address this issue . 
