This trail appears to be regularly used by hikers and snowshoes and was not posted.
Drive down mountain park rd and park at gate/intersection with Mt. Tom ski rd.
From here loop south around the field and follow the trail through the woods between the fence on the west side of the field and the reservoir until it joins the service road. At the north end of the reservoir, branch off of the service rd and follow a trail up hill to rejoin Mt. Tom ski rd. follow the road for a few hundred feet until you are at the base of the ski area. From here the runs of the closed ski area can be easily hiked up and skied back down.