In Berlin, I once stepped into a train half full with racist, neo-nazi skinheads. At the time, I was in the city conducting research into Berlin's politics of immigration, research that included investigating the politics of racism and discrimination. Few experiences presented the realities of this politics more vividly than riding home with the skinheads that night. Of course, in reading the space of public transportation in this way, there is a risk of romanticizing it. The public space I find so fascinating is for other people a space of harassment and physical harm.
Berlin can again serve as an example.
During the worst of the violence against refugees and immigrants in the early 1990s, the attacks often took place on the city's public transit system. So frequent were the attacks and so incompetent and unwilling were the authorities to provide adequate protection for those under the attack, that many refugees and immigrants refused to ride on public transit after dark. In this case, sharing space in common was a source of life-threatening danger. And let's face it, even without extremes of violence, riding public transit can be a decidedly unpleasant, negative experience. In fact, it frequently is.