It’s not a derailleur, but some other method. Never seen it before. Anyone know what it’s called?
If I’m not mistaken its a Shimano Nexus. I have a Dahon folding bike that also uses this mechanism.
It is indeed, http://taipei.youbike.com.tw/en/f41.php
Personally, I’m quite impressed with the Down tube: A smiley curve that is solid and easy to straddle upon.
Internal gear hub