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I have a definitive list on the topic that I updated just this month. I've got stuff for the other Anglosphere countries, too.
In short, the best ones in the Western US (and the whole country) are the surviving Denver & Rio Grande narrow gauge lines in southwest Colorado (Cumbres & Toltec and Durango & Silverton); and the best one in the Eastern US is the Stasburg Rail Road in southeast Pennsylvania, which also has a historic ridable miniature steam railroad at its main station, and a major static exhibit train museum across the street.
