Alison Krauss’ debut with Union Station is the most resolutely traditional album in her catalog. It’s almost as if the 17-year-old prodigy had to earn her stripes from bluegrass purists before she could expand the boundaries of the genre. Two Highways is everything classic bluegrass should be, with playing that is as sharp as a tack, but delivered with the home-cooked sweetness of cornbread. “Wild Bill Jones,” “Windy City Rag” and the title track should be enough to blow the spokes off any bluegrass diehard’s standards. The band can raise a ruckus, but lest the listener forget, this is the album that brought to the fore Krauss’ voice, an instrument as powerful and delicate as a bird’s wing. Her vocals elevate the swaying, unadorned ballads “Lord Don’t Forsake Me” and “Teardrops Will Kiss the Morning.” Even as the band proves its traditionalist mettle, there are subtle hints of pop melodies and modern harmony, particularly in “Here Comes Goodbye” and a surprisingly splendid cover of the Allman Brothers’ “Midnight Rider.”