Sometimes, I am worried, and dare not challenge young colored males in groups.I commend you for your civic mindedness. I think western societies can be rather unruly, so you got to be careful, be wary of situational variables so you can protect yourself. In an orderly and compliant society like SG, anti-social behaviour on public transport or in any public space cannot be tolerated.
They made the doors twice as wide as the older trains, but too often, people in groups of 2 to 4 would block the exit while they are chatting notwithstanding that there is lots of space in the trains.