It is a two way street, so you have to stop and make sure no one is coming. It does have a sidewalk under it, mainly because the TransAmerican Bike Route 76 goes down 4th St.
It's looks to be made of wood and steel, with grates covering the spaces, stone walls underneath, and a tiny wire fence on the edge.
ALL PICTURES HERE -- http://www.flickr.com/photos/94611454@N ... 451256521/
Below are some from the link above:

One Lane Underpass by Signals Unlimited, on Flickr

One Lane Underpass Sign by Signals Unlimited, on Flickr

One Lane Underpass by Signals Unlimited, on Flickr

9'-2" One Lane Underpass by Signals Unlimited, on Flickr

One Lane Underpass by Signals Unlimited, on Flickr

One Lane Underpass by Signals Unlimited, on Flickr

One Lane Underpass by Signals Unlimited, on Flickr

One Lane Underpass by Signals Unlimited, on Flickr

One Lane Underpass by Signals Unlimited, on Flickr

One Lane Underpass by Signals Unlimited, on Flickr

Railroad Bridge No Pedestrian Trespassing Sign by Signals Unlimited, on Flickr

One Lane Underpass - Above by Signals Unlimited, on Flickr

One Lane Underpass - Above by Signals Unlimited, on Flickr

One Lane Underpass - Above by Signals Unlimited, on Flickr

One Lane Underpass - Above by Signals Unlimited, on Flickr

One Lane Underpass - Above by Signals Unlimited, on Flickr

One Lane Underpass by Signals Unlimited, on Flickr

Forgotten Railroad Equipment by Signals Unlimited, on Flickr

Forgotten Railroad Electronic Part by Signals Unlimited, on Flickr