As far as we could tell, we traversed the whole thing. It was fairly contiguous, with only a few short bits on road, until the Air Line appeared to end in Mechanicsville. From there, we rode a mile or so to Putnam, CT for lunch. Other maps on RidewithGPS seem to indicate it goes 80 miles, all the way to south of Hartford. But at the present it would require more research to discover where it might actually pick up again, and how much of the rest is just a dream. Kudos to those who have mostly protected this corridor from business development over the years, and even greater praise to those who had the vision and took the initiative to lay infrastructure for recreational purposes. There is still a lot to do. The most amazing improvements were in Blackstone, where they built beautiful new bridges and laid down pristine asphalt. There were also long sections in MA and CT where finely crushed gravel has been compacted to a very rideable surface. However, for those considering doing the whole thing, note that there are uncomfortably long sections that contain all sorts of different challenging terrain. This includes mud, sand, swamp, rocks, sticks, vegetation, and very rough gravel. This meant that my 700x32 tires at 50 psi were barely adequate for traction and shock absorbtion. I'd prefer at least 40's, perhaps 50's, and maybe even a mountain bike with shocks — if you don't mind pedaling the extra weight over that distance. There were also short bits that were extremely steep where it rose or dropped to cross a road, so high gearing was necessary unless you don't mind a little walking. The grade on the vast majority is practically zero of course, like other rail trails.