Approximately once in a generation, there comes a bluegrass band that so perfectly integrates tradition and innovation that the listener can only sit back stunned and slack-jawed. In the 1950s and beyond, it was the Country Gentlemen. In the 1970s and beyond, it was the Seldom Scene. These days, it's Blue Highway.

Formed in 1994, the five-member Blue Highway has maintained the same line-up (except for a short departure a few years ago, followed by enduring return, by banjoist Jason Burleson), which probably helps explain why with each succeeding album the band, whose first recording It's a Long, Long Road (Rebel, 1995) harvested a fat crop of rapturous reviews, just gets more thrilling. Through the Window of a Train feels practically flawless, though not in the way flawlessness can devolve into bloodlessness. There's impressive instrumental and vocal virtuosity here, but there's also plenty of soul.

Bluegrass bands tend to be either traditional -- reflective of the foundational sounds of Monroe, the Stanleys and Flatt & Scruggs -- or progressive. Lots of good music has been made on both sides of that equation, though in the latter instance the results sometimes transcend anything that can meaningfully be called "bluegrass." As often as not, it's acoustic string-ensemble music that feels neither Southern nor rural and whose point of reference is as likely to be urban pop and jazz (or even rock) as the fusion of older Appalachian folk music and mid-century country that ordinarily defines the genre.

The 1960s folk revival saved bluegrass's life. Redefined as folk music, which allowed it to survive its brief moment in the country-music mainstream, bluegrass has preserved both mountain and city folk-revival music and brought them to fans who did not grow up with either. Like the Country Gentlemen and the Seldom Scene, Blue Highway is positioned as much in the folk-revival impulse as in countrified musical culture.

Consider the songs, in this case all originals, nearly all of them ballads in the venerable story-telling sense. (On the other hand, "Just Another Gravel in the Road," which does not answer to the just-stated description, sounds like the sort of song the immortal Carter Stanley would have composed on one of his even-better-than-usual days.) Written by the band's guitarist Tim Stafford with non-member Wood Newton, "Two Soldiers" borrows the title, albeit not the plot-line, of a wrenching Civil War-era ballad (covered by Bob Dylan, among others) to tell an Iraq War tale of "two soldiers no one wants to see" -- the ones who show up at the door to tell a family of the son or daughter who "ain't coming back." The melody and arrangement are contemporary, but as with several of the cuts here, the arrangement is folk-like and skeletal, with just two guitars (Stafford and Burleson) and mandolin (Shawn Lane), with some faint percussion -- as if at the hand of a ghostly drummer -- by non-member Tony Creasman. The effect, at once beautiful and shattering, sobers and unsettles.

"Soldiers" is arguably the finest song of the 11 (plus the Burleson-penned instrumental "The North Cove"), but the competition is steep, with no weak pieces slowing the momentum. "My Ropin' Days are Done" (by Stafford and Bobby Starnes) achieves some of its power through its strikingly imagined incorporation of a melodic quote from "Streets of Laredo." Bass player Wayne Taylor's "Homeless Man," about a disturbed veteran forced to live on the streets of Los Angeles, is set to another memorable tune, memorably sung. If there is no mistaking the political subtext, its impact is yet enhanced by the song's refusal to bludgeon the listener with the story's moral.

The praise could go on and on. I haven't even mentioned Rob Ickes's extraordinary way with the Dobro. I haven't stressed sufficiently how superior the vocals are, how crisp and tasteful the picking, how creative and robust the arrangements. Blue Highway all but defines what fresh and original, yet firmly rooted bluegrass, sounds like in the early 21st century.