THIS SIDE OF BRIGHTNESS.<i> By Colum McCann</i> .<i> Metropolitan Books: 289 pp., $23</i>

1915. The tunnel advances 12 feet a day below New York’s East River. In front of the steel safety shield, a pinhole opens suddenly in the muck where four sandhogs are shoveling. Compressed air has kept their excavation from caving in on them; in an instant its pressure widens the hole to the size of a ball, a platter, a man’s body. It blasts the hogs up through the riverbed and the icy water above it. They break the surface and erupt cartwheeling in a brown geyser 25 feet high.

The explosive launch to Colum McCann’s novel, like that of a space probe, takes the reader to some far and uncanny places. It is hard to encapsulate “This Side of Brightness” or to convey its author’s accomplishment in combining so many different elements to such effect.

It is partly a story of the men who dug and blasted New York’s tunnels and of the high-steel workers who turned horizontal astonishment upon its vertical end, balancing hundreds of teetery feet above the streets to subdue the swinging girders and bolt them together into skyscrapers. Told with gripping realism and subtle detail, the facts--history researched--glow like jewels.

“Brightness” is also a moving, beautifully written account of three generations of the racially mixed family of Nathan Walker, a black man from the Okefenokee Swamp who rises to the perilous tunnel position of “front hog” and, having survived the East River blast, goes on to work underground for the next 30 years. His life curves arduously up and then declines, as if time were the geyser that lofted him in the air and dropped him back in the river.

In part, the fall comes from the brutal discrimination deployed against blacks in New York in the ‘20s and ‘30s. In Nathan’s case, it has a double charge because he married a white woman: Eleanor, the daughter of the one of his three fellow sandhogs who didn’t survive the blast.

Everything is hard for the couple: finding a room to live, shopping--Eleanor is not allowed to try on a hat because her husband is black--and going to church. Her Irish Catholic parish snubs her; finally she gets herself a Baptist baptism in Harlem.

Nathan, whom McCann depicts with sweet particularity, is brought down in other ways as well. He is a gauge of New York’s half-century shift from a place of vital physical growth, where manual skills and endurance were needed and a workingman could show his grandchildren the bridges and tunnels he built, to a place where blue collar has no place or meaning.

No longer was there a real working class. To be poor was no longer to stand on the lower rungs of the ladder. It was to be derelict because rungs and ladder were gone.

Nathan’s long, loving marriage to Eleanor, his Irish American wife, ends when she is run over by the neighborhood drug dealer flaunting his brand-new car. Their son, Clarence, once a promising student and now a Korean War veteran, kills the dealer and a policeman who tries to arrest him; he flees south, and the Georgia police shoot him dead.

Bereft of all that his life had achieved, Nathan journeys to collect the body. It is 1955--McCann drags us back through history--when an Atlanta policeman can greet him with the words: “I got myself one of your kind in my family tree. Just a-swinging away from the highest branch.”

Clarence’s own son remains; Nathan cherishes and brings him up. The boy inherits his grandfather’s building passion; not for tunnels but for skyscrapers. He works the top spot in his high-steel team; he prospers, marries and has a child. He reproduces Nathan’s early success, and it seems that now, in the 1980s, a black man will be able to hold onto it.

Then Nathan is killed--McCann, whose symbols can be clangorous, has him crushed by a subway train while visiting one of the tunnels he built--and Clarence breaks down and is lost. It is not race this time. The author has a cause in mind, though he lays it down rather imperiously and with some cloudiness of connection. It has to do with the lost tradition, comity and human structure of a modern city.

Those who have built New York are ground up and cast aside: This is McCann’s theme. He develops it for the most part with authentic drama, despite occasional melodramatic inflations. In any case, the passion he brings to it gives the book its compelling energy. Its art comes in part from the inventive portrayal of Nathan’s valiant life and unquenched sensibility. In part, it comes from an original and disconcerting construction.

“This Side of Brightness” is two books that converge, written in drastically contrasting styles and alternating chapter by chapter. The first starts with Nathan in 1915 and works forward through his life. The second starts in the present and works back. It portrays the life of a man nicknamed Treefrog, one of hundreds of half-crazed derelicts who molder in New York’s subway and railroad tunnels, venturing out to scavenge or beg.

The story of Nathan and his family is written with poetically detailed realism and narrative sweep. Treefrog’s story is told with a bleak modernist unsettlement. Instead of recognition, its details build estrangement. Instead of locating us, they lose us.

We read of Treefrog’s cave 20 feet up a catwalk from the tracks, a grating that admits a little rain or snow, the pirated electricity that another derelict wires up for the community, the perpetual cold, the fires made from leaves that Treefrog bags in his above-ground sorties, his doomed love for a drugged-out waif, glimpses of neighboring cave dwellers who have dropped down from life.

There are no stories underground. The element that makes them possible does not exist. The tunnel derelicts, whose world could be our image of nuclear winter, exist like the damned in Dante, in “an air uncolored by time.”

As we alternate between Nathan’s story and Treefrog’s frozen inferno, our sense of disconnection, augmented by the difference in writing style, seems complete. Then about halfway through, pinholes open like the one that sucked Nathan up through the river. A touch of his life slips into Treefrog’s, a hint of who Treefrog once was slips into Nathan’s. Finally we learn Treefrog’s identity, and the connection is made.

It would be unfair to say more; only that the tunnels, which for Nathan were a great enterprise he was part of, have become a judgment on the world that uses them. It is a world that came to defraud the laborer not only of his hire but of his labor, employing its splendor to serve the purposes of a mean city and keep its human discards out of sight.