"With the way the North Koreans are suffering these days, you would think an assasination attempt would be tried."
I got flamed somewhere else for this, but I believe the rail car explosion was just that.
Don't feel bad. I was initially flamed for leaving open the possibility of such (I felt that the timing of the explosion relative to Kim's train schedule -- always bounded in tight secrecy to prevent detailed forecasting -- was an unusual coincidence).
Analysis and reporting over the past year, however, has made that scenario more likely (I don't have the references on hand, but see, for instance, Rebecca McKenna's "NK Zone" site.