After I commented on the cleverness of the Da Vinci tandem drivetrain, the guy manning the booth told me "we've been here 17 years in a row". I responded, "but this is my first time here." So this may not be new, and I've likely seen it before @ NAHBS, but in any case I considered it cool. The goal is to allow the stoker to pedal independently of the captain, independently coasting, although still constrained to the same cadence. A side effect of this is the pedals can come out of synchronization, although the stoker could time reinitiation of pedaling to synchronize as desired. Despite their longevity they don't seem to have caught on, as most tandem riders seem content to remain locked together with a fixed timing chain.
Da Vinci tandem drivetrain
Da Vinci drivetrain, stoker close-up
Another curious drivetrain was the "string bike". This was quite remarkable, converting a circular motion on the pedals to a piston-like motion of the strings, the strings rotating the rear hub. Of course nothing's actually new about this: it's similar to what happens (in reverse) to an internal combustion engine, where linear piston motion rotates a drive shaft. I am not sure about the motivation for using a string rather than a chain to drive the reart hub, but if nothing else it may be cleaner.
String bike drive-train
Then there was the 14-speed Rohloff hub. I've seen this before, at NAHBS. It's truly an amazing piece of engineering. The efficiency is fairly good, as well, approximately 96% to 98% in according to their data depending on the selected gear. The most obvious penalty is mass: the thing is heavy. But for loaded touring, the weight will be hardly noticed, and the advantage of having a fully encapsulated system protected from the elements when riding in remote areas with potentially little access to bike shops is surely attractive. They had it attached to a Gates belt drive, which replaces the articulated metal chain in conventional drivetrains. This is cleaner but if a Gates belt breaks you need to have a hard-to-get replacement, while with a chain you can replace or remove the broken link pair. So if I was touring I'd make sure I had at least one extra.
Rohloff 14-speed hub schematic
Rohloff efficiency, compared with conventional drivetrain
Finally there was this ordinary which, as is typical, has a direct drive. I thought this was cool enough that I took a photo.