This novel works for a couple of reasons. First: Roberts could write as well as any literary novelist:
At three in the afternoon the engine sheds were already gloomy with the coming night. Light, blue and vague, filtered through the long strips of skylights, showing the roof ties stark like angular metal bones. Beneath, the locomotives waited brooding, hulks twice the height of a man, their canopies brushing the rafters. The light gleamed in dull spindle shapes, here from the strappings of a boiler, there from the starred boss of a flywheel. The massive road wheels stood in pools of shadow.
Through the half-dark a man came walking. He moved steadily, whistling between his teeth, boot studs rasping on the worn brick floor. He wore the jeans and heavy reefer jacket of a haulier; the collar of the jacket was turned up against the cold. On his head was a woolen cap, once red, stained now with dirt and oil. The hair that showed beneath it was thickly black. A lamp swung from his hand, sending cusps of light flicking across the maroon livery of the engines.
Secondly, Roberts uses linked short stories to tell an epic tale in an understandable manner. Some have criticized Robert's use of linked short stories, as being too simplistic or muddled, but I think they may not understand that linked short stories provide a whole greater than the sum of the parts. No only is there a story arc in the individual stories, but a greater arc when the stories are taken together. Linked short stories are one of my favorite forms of storytelling. Check out Love Medicine by Louise Erdrich, Annie John by Jamaica Kincaid or The Things They Carried by Tim O'Brien. They're all great literary novels.
Oh yes, a word of warning. I didn't perceive the book as being anti-Catholic, but devout Catholics may not like how the Church is portrayed. Hey, I'm a lackadaisical Unitarian, what do I care?