"Shout Up a Morning" is a huge, heroic musical about the life of that mythical steel driver, John Henry -- "a man so big he got two first names."

The outsized proportions are established from the start, when the lights go up on a three-story slab of stone that set designer John Arnone has positioned dead center on the stage of the Eisenhower Theater. In bold, abstract terms, this is the forbidding West Virginia mountain through which workmen for the C&O Railroad are painstakingly chiseling a tunnel. But it's also a symbolic barrier, keeping a ragtag clan of wandering blacks, newly liberated by the Civil War, from happiness in the promised land of far-off Mississippi.

John Henry, as you may remember from the folk song, is the "hammer man," who pitted himself against a steam-driven drill and won. In the view of Paul Avila Mayer and George W. George, who wrote the book for "Shout Up a Morning," he is more -- an angel of the Lord, come down to Earth to forge unity among his people, instill them with pride and deliver them to freedom.

The musical, born at the La Jolla Playhouse in Southern California and brought here under the auspices of the AT&T Performing Arts Festival, has so many virtues -- from the dignity of its themes to the strapping performances of the large cast -- that one is a bit dismayed to find it fairly laborious, especially in its early stretches. There is not a lot of story to tell, but what little there is stretched out for almost three hours. It's not just the tunnel that inches forward; "Shout Up a Morning" plods much of the way to its uplifting finale.

The score by Julian (Cannonball) and Nathaniel (Nat) Adderley is not without varied beauties -- hints of calypso infiltrating plaintive strains of jazz, which, in turn, give way to the rolling majesty of the black spiritual. When they're not forcing such rhymes as "gullible" and "full of bull," the lyrics by Diane Charlotte Lampert have a homespun poetry. And the full-throated cast sings out with the fervor of the blessed.

But the evening's assets tend to remain curiously self-contained. Neither the saga nor the stark staging by Des McAnuff generates much cumulative power. The show bears an unsettling resemblance to those vast murals that once decorated our courthouses and depicted in a cluster of muscular but static images the history of a county, its people and its industries.

You don't want to write off "Shout Up a Morning," however. Somewhere in its heavy mass are the seeds of nobility and a heartfelt simplicity.

When, for example, the beguiling Leilani Jones, bent over a washtub, sings of her dawning affection for John Henry in a plaintive ballad, "Gonna Give Lovin' a Try," the stage begins to glow with warmth. And "A New Star Risin'," her realization that she is carrying John Henry's child (and maybe a new Jesus), weds a pristine sense of wonderment to the warmth.

As John Henry, Michael Edward-Stevens is a pillar of strength -- he may not be seven feet tall, but he sure looks it -- and his robust voice adds several cubits to his height. What makes him so persuasive as a savior, though, is the clumsy gentleness. He can command the mountain to quake -- and the mountain does -- but it's the spontaneity of his broad smile that lights up the dark night and ultimately endears him to us.

"Shout Up a Morning" has its villains -- the white bosses and foremen of the C&O Railroad, who pay $1-a-day wages, and then, when John Henry pressures them into doubling that miserable sum, fire the workers and bring in a steam engine to take their place.

Mostly, though, the musical focuses on the members of the clan -- Jack Moses, the grizzled patriarch (Nick LaTour); Jassawa, the simpleton (Stuart K. Robinson); Grandy, the juju woman (Leila Danette); and the assorted husbands, wives and children (Stewart F. Wilson-Turner and Charlaine Woodard chief among them), who are struggling to stay together, while life's hardships are forcing them apart.

The sassy Woodard gets to preside over the show's spunkiest moments, in which the womenfolk, fed up with disintegrating morale in the clan, instruct the men peremptorily to "Grind Your Own Coffee." The rowdy down-to-earth song is noteworthy in another way, too. It tends to counter the tendency of "Shout Up a Morning" to subscribe to its own mythical stature.

The linear story line eschews all but the most elemental conflicts; the plot is, in fact, largely summed up in the show's second-act production number, "The Ballad of John Henry." But the authors are so concerned with underscoring the biblical, sociological and economic implications of the tale that its simple humanity is further diluted. When you set out to make characters larger than life, they often end up looking smaller.

I'm not sure the monolithic mountain helps all that much, surrounded as it is by orange metal scaffolding, from which the white overlords gaze down on the wretched workers. It may be symbolically appropriate. But rather like a severe, disapproving dowager, it discourages intimacy and throws a pall over a musical that is really about hope and rebirth.

* "Shout Up a Morning," which runs through Aug. 9, certainly has all the raw materials to become a success. But it will be a far bigger show, I suspect, when its creators decide to cut it down to size. Right now, it's trying so hard to stand tall, you can feel the ache in your toes.

Shout Up a Morning, music by Julian and Nathaniel Adderley, lyrics by Diane Charlotte Lampert, book by Paul Avila Mayer and George W. George. Directed by Des McAnuff; choreography, Dianne Ruth McIntyre; orchestrations, Kirk Nurock; sets, John Arnone; costumes, Susan Denison; lighting, Richard Riddell. With Michael Edward-Stevens, Nick LaTour, Stuart K. Robinson, Leila Danette, Edwin Battle, Charlaine Woodard, Ellia English, Mary Bond Davis, Leilani Jones, Stewart F. Wilson-Turner, George McDaniel, Michael Champion. At the Eisenhower Theater through Aug. 9.