The Tarrytown Lakes Extension is a pleasant ride, although not completely paved. I rode it to the then-current end of the trail last summer, where there was a bridge without a deck and orange snow fencing. Looking at aerial photos, I see no reason why the trail couldn't be extended, so maybe it will be?
There's also a bit of railbed which is unpaved, but seems to currently be used as a footpath, the Mohansic Branch. They were going to build a state mental hospital in what is now FDR State Park. Because of concerns about development so close to Croton Reservoir polluting NYC's water supply, they abandoned the hospital construction. I am unsure about whether the railbed actually saw rails. I've seen claims that it did, and that they used it to bring construction supplies in. I've also seen claims that it was graded north of NY-35. It takes off of the trail in Yorktown Heights just south of Hanover Street. Followed Downing Drive to NY-118 where it crossed at grade and continued to Baldwin Road.
Look here: http://www.openrailwaymap.org/?lang=en& ... e=standard
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