It's so efficient that barge transport is still preferred to rail where a navigable river is available.

It's efficient in the sense that if you compare kgs of goods per km, but it's not efficient when you consider the great distances ships travel versus producing goods domestically, just to put those same goods on rail and trucks anyway.

If you consider the efficiency of shipping finished goods versus shipping the raw material (and, possibly, labor) necessary to produce the goods domestically, how does that affect the comparison?

This statement seems to assume the same shipping of raw materials isn't taking place elsewhere.

Ore goes from the mine, to the forge, the raw steel goes to the manufacturer. That probably requires a bit of shipping, doesn't matter which country it takes place in.