I believe you are referring to a gantry crane to unload container ships, which Walthers does not produce. AFAIK, the only available cranes of this type in N scale are available from Searails (
http://www.searails.com ). I believe these cranes are made from resin and have some etched brass parts. N scale container ships pop up on that popular auction site with relative frequency (also resin, I believe). I passed on both and opted to scratchbuild myself a crane and container ship, although both are still not finished.