Traditional, old-time mountain music, aka bluegrass-folk-gospel, was an essential part of the development of virtually every branch of American music. But it was largely seen as a quaint, uncommercial ancestor of country until 2000, when that view was blown to bits by the unexpected success of the bluegrass soundtrack to the Coen brothers film, O Brother Where Art Thou, followed by the Down From The Mountain tour and subsequent album and concert DVD.

Singer/fiddle player Alison Krauss would never be so bold as to take credit for her part in the resurgence of old-time bluegrass, even though her show-stopping appearances on both albums were crucial. And she was shocked at O Brother's massive success.

"Oh yeah -- not because I didn't think the music was great, but because I didn't think it would ever get out there," she says. "It was like a dream come true to see people that excited about bluegrass. They got a chance to hear it, and it sounded like home."

Perhaps even more important than her O Brother contributions, though, is the fact that Krauss has been quietly building a hugely successful career -- 17 Grammys, numerous CMA and International Bluegrass Association awards and chart-topping sales -- since she was 14 by simultaneously preserving the old styles and gently pushing them forward with the help of her longtime band, Union Station.

"It's not a conscious thing," she says. "You get a song you like and you go with it. I'm the one who loves to find the material. I bring most of the songs to the group, and we all choose them and work on them. We've been very lucky with finding material that is inspiring to us, and all the guys have their own sound that's so identifiable when they play. And since we've played together for so long, the band has a sound of its own."

Lonely Runs Both Ways, the latest album from Krauss and Union Station -- Dan Tyminski, Barry Bales, Ron Block and Jerry Douglas -- ingeniously alternates modern country songs like Gravity and Restless with traditional bluegrass hoedowns like Del McCoury's Rain Please Go Away. Four of its tracks, including Restless, are by Nashville songwriter and Krauss fave R.L. Castleman.

"When I'm gathering stuff for everybody to hear, there's usually one standout song and you pick everything else around it," Krauss says. "This time it was Restless. It's a happy-sad song, and when I heard the lyrics, I could hear myself in it.

"When I met R.L., I thought, 'Where have you been all my life?' I felt as if I'd just opened the door to something really special. I got some songs from him, including Forget About It, the title track of one of our records, and they were unique. They're not Top 10 country songs -- even though I think they should be -- but they work for us.

"When I hear them I can't believe they haven't been cut by somebody who's on the radio all the time. They're just so appealing."

Lonely Runs Both Ways also includes a real rarity -- This Sad Song, a track co-written by Krauss herself.

"That was a mistake!" she says. "I wrote it as a teenager in the back of a van for fun, and Dan and Barry always liked it. I fought recording it for years, but they said, 'Shut up, let's just do it.' "

Krauss has been touring since she was a teenager, but she says her choice of genre saved her from burning out too soon.

"I was playing a form of music that wasn't mainstream, so I didn't grow up in the spotlight," she says. "I'm glad in a lot of ways that I've had a slow career. We've gotten better as a band, and our best records are the later ones. This is the most successful year we've had. So I'm happy things have gone the way they've gone."

Alison Krauss and Union Station play Massey Hall Saturday.