You can find out more about wheels here:
http://sheldonbrown.com/wheels/index.html

and about tandems here:
http://sheldonbrown.com/tandem/index.html

Though this forum is an excellent resource for all things tandem. The short answer on the wheel question is: its a size, the standard for road bikes. Mountain bikes typically have 26" wheels (smaller diameter than 700c) with a wider rim for wider tires.

Regarding the raleigh on a trail. Depends on the trail. We ride our road tandem (entry level cannondale) on gravel 'rails to trails' paths. They're fine gravel, but a pretty hard surface, so it works pretty well. I'm leery of loose gravel, sand and the like.