Here are the two underpasses that have been recently built, as part of the St. Vrain Greenway extension to Sandstone Ranch. Supposedly this will be all finished this year (2009).
The first crosses under 119th Street, not too far from the Sugar factory.
The second goes underneath the much busier Weld County Road 1 (border between Boulder and Weld Counties):
The tree is still there, although I guess it is dead. Wasn't sure back in March.
For wildlife conservation reasons, no dogs, including those on leashes, will be permitted on the Greenway east of this bridge. There will be signs, probably near this location, and fines for violators.