This was intended to be an easy recovery obtained while my father was picking up custom-made windows at nearby Madison Vinyl. Rich and I walked west along the tracks about 400 feet from the crossing at Dingman Hill Road and found the obvious bridge over a small creek. The majority of the upper surface of the abutment was exposed, but we couldn't find the mark. We scraped off the small amount of mud and gravel that had accumulated, but still, no mark was evident.
The abutments of this bridge have lower "shelves" that would serve as another good setting for a benchmark (even though the description seems to imply that the benchmark would be set into the top surface). But thanks to the brutal winter we've had and the extra shade provided by the bridge deck, the lower shelves are still covered with snow piles encased in ice. There's no way we would be able to clear that much ice and check the lower shelves.
The datasheet doesn't contain much additional information to help us determine the mark's location. The bridge and milepost numbers in the description do not match the numbers we found at the site. I ran west on the tracks for a distance to confirm that there are no other bridges or culverts in the vicinity. Additionally, the topo map indicates a benchmark at this location. A conversion of the State Plane coordinates (using the NY Central zone, 3102) provided by the ACE in their recent recovery places the mark in another county, so I'm not sure if there's an error or if we're calculating incorrectly, but we used the conversion tools available at NGS.
We'll have to return and search again!