No matter how the music industry changes, strong songwriting and alluring harmonies never go out of style. The Indigo Girls’ Amy Ray and Emily Saliers have never let fashion dictate their approach or vision. They rode a ‘back to basics’ wave in the late ‘80s to bring their earnest folk music to a mainstream audience and while they allow for modest production touches to augment their coffeehouse folk vibe, they’ve never lost sight of the song. Lisa Germano adds complimentary mandolin and violin, Jane Scarpantoni provides cello in spots, and Ray and Saliers fingerpick with admirable precision, but it’s the sweet harmonies and swirling melodies of “Power of Two,” “Least Complicated,” “Touch Me Fall,” and the slow procession of “Dead Man’s Hill” that prove this group’s personal depth. “This Train Revised” stretches beyond the band’s usual relationship and better world anthems to confront the evils of the Holocaust, ending the album on a warily socially conscious note.