I haven't heard the term "The Belt" before either, but I did know what it referred to. While I wouldn't recommend that someone take the Blue Line late at night, and it's a bit of a weird train - and I wouldn't recommend wandering around there - I've taken the Green Line a lot and I've also had medical appointments in South Central and I've been fine. Most of the people on the Green Line, at least during the day and early evening, seem to be regular people. There are a lot of people who work at LAX who always use that train. Most of the street harassment I've encountered taking mass transit has been in "safer" places like Santa Monica, oddly enough.