I'm pretty sure Woodbine is the longest but I wonder if my eyes trick me at my local OTB.
Old School got it, it's funny watching first time jocks hit the top of the stretch there and start pumping on their horse, then to see them with nothing left at the 1/8th pole. The local riders or Euros are always good bets there. Belmont is the longest for the dirt though, if that's what you were looking for.
Belmont Park is a major thoroughbred horse-racing facility located in the hamlet of Elmont, New York in Nassau County on Long Island (just outside of New York City). Its mile-and-a-half (2.4 km) main track is the largest dirt course in Thoroughbred racing. It first opened May 4, 1905.
It is indeed the turf course at Woodbine.