You could take the train, Amtrak, to Dunsmir, which is about 9 miles south of Mt Shasta. Greyhound bus also goes from Sacramento to Shasta.
If you rent a car, it's a pretty straight forward drive to get to Shasta and there are a few fun stops you can make along the way ... Gazelles for lunch or the Olive Pitt in Corning just to name two.
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