A railroad once went to the east of Van Cortlandt Park and to the east of what is now the Saw Mill River Parkway. Long abandoned, it became a trail. It has now been paved at the Westchester County line, but remains unpaved in the Bronx (although there’s a battle on that going on).
So there’s a nice stretch of trail in the Bronx which one gets to by heading from the south end of the fields of the Van Cortlandt flats toward the golf course, and Van Cortlandt Lake. Turn left before reaching the road to the parking lot and you’re on the trail. There is a way to connect to the OCA but it is tricky, basically involving taking a sharp turn to the east of the golf course and climbing a hill. You go past the staircase that used to lead to it, which is closed, and you’ll see a worn stretch to your left, which you climb up. Head north and you hit a newly-paved stretch and then you’ll see an unpaved trail to your left (after a bit), and that’s the OCA. HERE’S AN APPROXIMATION OF THIS CONNECTION.
One can make a loop with the OCA by heading north on the OCA and then turning left when it ends in Yonkers, down the hill, and the around the northern end of Tibbetts Brook Park, where one will eventually see a right turn to the South County Trailway (the Putnam Rail Trail that is now paved in Westchester) and then heading left, and south, where you pick up the unpaved trail at the City line.