I found some posts or rumors about it getting closed down due to abuse but also heard it was closed due to lack of players and expiry of domain.
The last bit is kind of invalid as there's no reason why you can't run Japanese or any other server on .com domain like jp01.rail-nation.com.
I wonder how little
paying player is too little?
If anyone has a link to an official announcement post regarding the closure of JP01, it'll be greatly appreciated.