Now I know why I had a hard time getting back home last night (but not like the other time, only had to wait about 15 minutes at Takadanobaba station). From Japan Today:

A Seibu Ikebukuro passenger train derailed late Tuesday afternoon after hitting a driverless truck that entered into a rail crossing in Tokyo. The incident threw the evening train commuter traffic into disarray and affected 220,000 passengers, Seibu officials said.

The police arrested the driver of the 2-ton truck, a 21-year-old man named Yutaka Nishikawa, on charges of professional negligence. (Kyodo News)