Yes, ships are far more efficient at moving containers in bulk quantities.
What they are not so good at, however, is moving them quickly. Trains are far better for that.
CN has managed to capture a not-so-inconsequential portion of the huge Mediterranean-to-Midwest shipping market simply by virtue of having the furthest eastern container port in North America. Depending on the other ports-of-call, CN can save shippers up to 2 days of transit time. They even have a service that allows shippers to ship from China to Europe, using the CN system as a land bridge. While not as fast as shipping through the Suez, it is faster than shipping via the Panama Canal.
All that being said, Sydney does not make a whole lot of sense to me as a container port. It wouldn't offer much of a substantial time savings over sailing to Halifax. Neither of the Halifax container terminals are operating at over capacity, and thus are not suffering from the huge delays seen elsewhere. And there's only one rail line in-and-out, who's connection is to CN - and thus the shipping companies would (still) be beholden to just the one railway for their service.
Dan