There's zero chance you'll ever see the Air Line reactivated for passenger service. Between Middletown and Putnam, it's curvier than the worst Shore Line segments, with hardly any tangent track. It also misses every population center (Hartford, Springfield, Worcester, Providence) worth a HSR stop. Even if the ROW were completely available, it would be worthless for any kind of HSR service.
The "Airline" train from East Hampton (Cobalt) east has existed for a while. In the past couple of years an effort has started to extend the trail west to Portland, CT. That would be from where line crossed route 66. Some of it has been finished.
BTW the line west of the route 66 crossing in Portland is still active with a couple of customers that requires the swing bridge to operate, maybe once a week.
