Intimidation. I dealt with that a lot when riding the bus in my city, was a problem back in 1998, was a couple of years ago, and from my understanding still is today. Perhaps my city just sux though.

http://wfpl.org/charges-dropped-tarc-stabbings-downtown-loui...

http://www.wlky.com/article/tarc-adding-security-measures-in...