The Fore River trail is butter smooth sand, wheel chair accessible. But the trail on the other side of congress (behind Maine Ortho) is mostly single track. A little climb, some fun stairs, and a very technical area with frequent mud and some interesting bridges.
Your best bet is to park in downtown portland and ride along congress street. Ride into the parking lot at Maine Orthopedic Center and look for the trail to start on the right side about 1/2 way back into their lot.
http://www.singletracks.com/item.php?c=1&i=5403