Road rage exists, it is just less visible, especially if you're not involved. It usually won't be a car directly ramming into another, although that happens sometimes, it's going to manifest as one repeatedly cutting another off or one tailgating really close, which you probably won't notice if you're not involved but is still very dangerous. Meanwhile someone shouting or punching someone is very easy to notice on a metro car, especially since your attention is up for grabs whereas when your driving your attention is on keeping your lane, watching for your turn, maintaining your speed etc.

Yes being trapped in a confined space can trigger violence in some but in others it may discourage violence as there's no escape from the repercussions. Meanwhile if your in a car and side swipe someone you can just speed off and as long as someone doesn't catch your license plate your home free. Also cars can trap you in a confined space if your in traffic, which does trigger a lot of road rage.

I'll admit I only take the metro about once a week, mostly get around the city by walking and bus and I commute by my one wheel and commuter rail, haven't owned a car for almost all of my adult life. The main threat I face when in transit is not from crazy people on public transit, it's from cars when I'm walking or one wheeling. I think drivers normalize the danger of cars and sympathize with them, but once you spend most of your time outside the protection of one you can begin to realize how dangerous they are.