Located in Bombadil country in Wisconsin it's said to be the first rail-trail conversion. It's still the most fun trail that I've ridden on.
The trail is 32 miles long (more or less) and passes through 2 intermediary towns. The thing that makes it unique are the tunnels that pass through the biggest hills. The trail goes for about 5 miles up a 3% grade to a tunnel and down 3% for about 5 miles again to the next town so in 32 miles you do that 3 times. 3% is a grade where your eyes tell you it's flat but your legs are saying otherwise. The uphills, while not too hard, are depressing because you feel like you should be able to ride faster. The downhills are a blast for the same reason.
I'm sure there are people who ride through the tunnels but I didn't see any. There is no way I was going to attempt it on our road tandem. The floors, which you can't see very well, are kind of rough and there isn't any real vertical reference to help guide you. I think that I might try riding it on a mountain bike with a big headlight.
Most of the riders who were on our tour avoided the trail in favor of riding a little longer route with some steep hills. I'm very happy that we rode the Elroy to Sparta trail. (Actually we rode Sparta to Elroy but who cares?) To me it was the highlight of our trip.