The Twin State Railroad (and formerly operated by many many other companies over the decades) was a branch of railroad line that operated between Whitefield NH and St. Johnsbury VT. Service was discontinued partially until the paper mill in Lunenburg closed in 1999, when the line ceased being used entirely. The railroad formally petitioned to abandon the line in 2004 but the tracks will continue to rust in place and fall apart until environmental impact issues are resolved and a salvage plan of some kind is established.
The bridge across the Connecticut River is no longer being maintained, like the rest of the line, but it's a pretty cool sight. It is currently possible to walk across the bridge -- there was even once a long-since-archived cache hidden in the middle of it -- but since the structure is not actively being being kept up, do so only at your own risk.
There is also a pretty decent trail path alongside the old tracks, so it is fairly uncomplicated to walk in from the VT side.