An ornate soap opera of the sort that has become a fixture of Sunday nights on PBS, the eight-part “ Marie Antoinette ” is a little like “Titanic”: We all know the subject’s destination. The question is how long it will take to get there. It’s not a brisk trip. Not usually a problem, if the parties are eager and the plumbing is intact. But the dauphin, Louis-Auguste (Louis Cunningham), is diffident to the point of paralysis and Marie spends much time worried if he’ll ever rise to greatness.

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