I don't really understand the ending of Gantz?

[Metalist00]

Well... I've watched Gantz (and I would NEVER watch it again!! NEVER!!! It's way pessimistic) but I really didn't get the ending... I don't understand the "meaning" of the last ~5minutes. He points his finger at the train and says bang... really, wtf is that...

Hoping I can get a good explanation...
Andrei