For context, the only recent rail disasters with death tolls in that range involved gas explosions or a tsunami. The alleged accident seems to have taken place at ground level too. The source is naturally an American book that claims to show "the real North Korea" and mostly cites random defectors in South Korea.
"rumored to be about 700" 
For posting all the anonymous reactionary bullshit that you can't post anywhere else.
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How did the train derail when North Koreans push it themselves?
They pushed it over too many dissenters that were tied to the tracks.