I've heard of many mountain bike trail building efforts within the City of Toronto, both sanctioned and unsanctioned, but this example of railway underpass / bridge building on the Don River is the most interesting one yet.
It consists of a wooden trestle about 15 m long and about 75 cm wide. It extends from one side of the bridge to the other. It is low enough for a biker but I had to duck my head when I walked along it. It is solidly built and creatively uses available space. the bridge is resting on poured concrete pilings that are sitting on rocks that were already there. From some of the comments I've read on the biker forums it took 40 bags of cement to build.
More details over at the Don Watcher blog.