hmmm...
Göttingen isn't on the Weser-Trail, but Bremen and Kassel.
More Infos on Vulkanradweg?
http://www.vulkanradweg.de/strecke-amp-partner.html
also
http://www.radreise-wiki.de/Vulkanradweg

Crossing Frankfurt while riding the Main-River Trail is easy, i agree.

I cycled Vulkanradweg 2 times and (as its on my way to my former home) and i love it.

ciao Jürgen

edit: Kassel on the Weser-Trail *lol*
Kassel is on Fulda Trail which flow into the Weser.