my conspiracy theory
I think it has something to do with the Chinese and North Korean talks of Nuclear disarmament. A terrorist group has been in contact with the North Korean government and has made numerous threats that they would cause large casualities if talks of disarmament are not ceased.
This terror group wants North Korea to keep their weapons in hope of overthrowing Kim Jong and starting their own government. They have ties to various terror groups throughout the globe. Believed that this group has ties to the North Korean military (kinda like a coup).
This explosion was just a show of their power. That they mean business. And that they could have taken out Kim Jong very easily. And if he doesnt step down and let their leader take over, even more terror acts will occur.
Either that or some idjit didnt switch the signals right and the trains ran into each other...