Not mentioned in the article: mobile phones can easily be used to trigger IEDs. That is the way it is often done. This may have been a reason the authorities in the DPRK were so nervous about having that kind of technology around.
I am still skeptical that this was really an assassination attempt. The explosion missed Kim Il Sung’s train by quite a bit, and caused much more collateral damage than would have been necessary to do the job. I think that it was an industrial accident, but one which played into the paranoia of rulers like Kim Jung Il.
NK's weapon's factories are nearby. One of the things manufactured is rocket fuel. So the station could have this kind of cargo stacked up or about to be loaded. The initial explosion could have set off much bigger explosion due to the presence of such chemicals.