Built this one myself, about 10 years ago. I put some 027 rail and Atlas HO bridge girders to good use. The actual bridge is wooden.
Crude, effective and the moat annoying project I ever did. Never again. Just write the check and never look back. I would buy one from Millhouse or Ross if I had to do it again.
BTW - mine was sized to the wheelbase of a Lionel SP AC-9 articulated. My biggest loco, and it shall remain so.