BOOK REVIEW : Simplicity, Dramatics Derail ‘Train Home’ : THE TRAIN HOME: A Novel <i> By Susan Richards Shreve</i> . Doubleday/Nan Talese; $19, 272 pages

It is, unquestionably, the best single wisecrack in the movie satire “This Is Spinal Tap”--the aging rock star proclaiming that the line between clever and stupid is very thin.

Well, there’s a similar fine line in the book world, the one dividing commercial from literary fiction for, ostensibly, literary fiction can be pretentious to the point of inanity while commercial novels, from time to time, are put together so well they transcend the genre.

It would be a mistake, however, to conclude that the distinction between literary and commercial fiction is therefore arbitrary. It isn’t--as “The Train Home,” the ninth novel by Susan Richards Shreve, makes clear in almost textbook fashion. It is an extremely readable book, the sort of novel one associates with beaches and sun-tan lotion, but it suffers from obviousness and insubstantiality. In a nutshell, the novel shows itself to be commercial rather than literary because Shreve actually intends to skim surfaces, to distill lives into implausibly simple constructs.

Shreve, for one thing, clothes her protagonists in unnecessarily alluring attributes, traits that feel conferred by the author rather than earned by her characters. Will Huston, an Irishman, has come to the United States to avenge the death of his beloved younger brother, then a child, during sectarian violence in Belfast 20 years earlier. Boarding the Metro in Washington, D.C., he encounters a young woman named Annie Blakemore who finds herself intensely attracted to him. She has a career and a family to take care of, but that doesn’t stop Annie from deciding, against her better judgment, to follow Will around.

It’s an intriguing, not unreasonable plot, but Shreve doesn’t stop there, heightening yet undercutting the story at the same time by giving Will and Annie overly dramatic lives. Will is not merely a vengeful man: He’s a playwright and actor with Dublin’s Abbey Theatre, he’s dressed up as a priest, and his target--the man who killed Jamey Huston--has become a British diplomat. Annie isn’t just an unfulfilled woman: She’s a Baptist, Texas-born opera singer who longed to be an angel as a child, has a deep, longstanding weakness for priests, and feels enormous guilt over causing the car accident that put her husband, Adam, permanently in a wheelchair.

Will’s anger, Adam’s disability, and the two main characters’ contradictory views of the priesthood appear, on paper at least, to be significant impediments to romance. In fact, of course, they are only straw problems: Will doesn’t slay his brother’s killer, Adam fails to exhibit a single redeeming characteristic, and although Annie’s fascination with priests lies in her desire “to make a confession and to be forgiven,” she isn’t the least disturbed to discover that Will is not a man of the cloth.

Fate, in short, obliterates the story’s many dubious moments. When Annie finally catches up with Will on a train to New York, they recognize their mutual destiny upon learning that Will had serendipitously looked after Annie’s children (named, insufferably, Nicholas and Alexandra) while attending the opera in the course of his Washington assassination junket.

In its bound galleys--the near-final version of a manuscript sent out by publishers to generate early interest in a book--”The Train Home” was initially described as “a perfect summer read for those who were captivated by ‘The Bridges of Madison County.’ ” And that’s very likely true--even though the publisher subsequently abandoned that description (it’s been blacked out on the galley’s back cover) and “The Train Home” is actually better written than Robert James Waller’s mega-bestseller. The central problem with Shreve’s novel is that her prose, unlike Waller’s, is clear and unpretentious, leading the reader to expect more, to believe the author has honed her language because she has something significant to say.

Shreve doesn’t, though, despite her half-hearted attempts to wring significance out of the fact that both Will and Annie are professional actors. Shreve borrows glamour for her protagonists rather than making it seem an unavoidable consequence of personal character, and “The Train Home,” as a result, doesn’t live up to its promise.