Here's a link to a nice video produced by a Scranton TV station. Look in the list of titles for "Steamtown Excursion over Viaduct."
http://www.wnep.com/Global/category.asp?C=112458
Note: It is also known as the Nicholson Viaduct, as that is the small town that lies underneath; but I think the DL&W called it the Tunkhannock Viaduct after the name of the stream that runs under it.