I've been on three of the longest rail-trails in America:#Katy Trail State Park—Missouri: 240 miles

Palouse to Cascades States Park Trail—Washington: 229 miles

#Cowboy Trail—Nebraska: 219 miles

Great Allegheny Passage—Maryland, Penn.: 150 miles

Soo Line Trail—Minnesota: 148 miles

Columbia Plateau Trail State Park—Washington: 130 miles

Paul Bunyon State Trail—Minnesota: 119 miles

#Flint Hills Trail State Park—Kansas 118 miles

The East Coast Greenway has 3000 miles of trails, 98; of which are in my home state of CT. It links to other rail trails in the state.

We often hike along sections of the Airline Trail which has lots of interesting signs telling about the history of the areas and the construction of the original trail. Originally lots of wooden trestles that spanned the gaps between hills and over water courses, most of which were filled in, some with viaducts as trains got heavier. it is interesting to think about the people who traveled this way, going to the Civil war, WW1, west, and if they ever came back home.

NY has the 360-mile Erie Canalway trail that runs from Buffalo to Albany. I've never done the whole trail but have travelled large segments of it.
That Erie canalway is also part of the larger 750mi NYS Empire trail network.