The conclusion is that the US just has more psychos. But the restraint of wrecking their vehicle to accost me holds them in line. On public transit they can accost me and nothing stops them.
Let's be real for a minute; the relative incidence rate of highway shootings is far lower than the relative incidence rate of violent crime on public transit in American cities.
My not-sufficiently-informed opinion is in agreement with yours. Do you have any studies that we can use to compare shootings versus violent crime rate on public transit?
A quick Google search revealed some data collected by the US government on the topic.