I'll come out of the closet, and admit it ... I'm fascinated by abanoned and partly-built transportation infrastructure. Cleared rights-of-way for Interstate highways that were never built, exit ramps that lead to nowhere, long-forgotten streetcar tracks poking up through the tarmac, and so on.
During grad school, I killed too many hours hunting down remnants of Buffalo's once-extensive interurban network. Rights-of-way are still there in many cases, and along one line the cantenary pole support bases are still embedded in the ground, stripped of their host structures in the late 1930s. One three-block long street in a lonely subdivision, with the grandiose name "Buffalo-Depew Boulevard," is all that remains of a once-busy rapid transit line that stretched from downtown Buffalo to the far eastern suburbs and beyond. Follow the dirt path beyond the short street, and you'll notice that all the railroad lines that cross it do so on sizeable bridges.
Sites like Forgotten NY, London Underground History ... can't get enough.