When the evening sun is strong, the south stair tower of the New Carrollton Metro-Amtrak parking garage seems for a long moment almost to catch fire, its four-square top ablaze with a glinting white-yellow light.

Commuters have grown used to the sight. First-time visitors, however, may pause to contemplate the phenomenon and figure out its cause--the combination of sun angle and irregularly shaped yellow glass tiles at the top of the tower.

A little daily miracle, in other words, with a rational explanation. The tiles are part of an artwork conceived by painter-sculptor Heidi Lippman and architect Ben Van Dusen, and installed by Italian artisans last year on the four stair towers of the massive, concrete garage.

I first encountered the work in person eight days ago while on an early evening Amtrak train from Union Station heading for Philadelphia. Photographs I had seen had not quite prepared me for the subtle, but lasting, firsthand impact.

The train slowed and then halted, and my wide window was filled with a thoroughly ordinary view--train platforms with shuffling passengers, the big parking structure, people walking to cars in surface parking lots, low green hills in the background. Autos speeding homeward along Route 50, just barely visible, gave a buzzing frame to the scene: America at the end of a workday in the late 20th century.

The mosaic panels did not exactly take command of this everyday vista--there are not quite enough of them to do that. But they certainly affected it--and me--strongly. Veins of color, snaking around the concrete spines of the stair towers, brought animation to the building's powerful geometric form. Smaller triangles of glass and stone, randomly placed on the column capitals and the Metro plaza clock tower, were grace notes in the late afternoon light.

I resolved then and there to get a closer view as soon as I could. Large public works of art, made to be seen at a distance, often do not repay up-close inspection. Mosaics, however, are different. The textures and colors of the small bits and pieces that make up the whole often have more impact at close range than from afar.

This proved to be emphatically true of the New Carrollton work. The variety and quality of the materials is phenomenal--glass pieces of many sizes and shapes along with slices of marble, striated agates, chunky granites and many other stones. The depth and range of colors is equally impressive--molten crimsons, iridescent blues, silver-flecked yellows, aqueous greens, spotted magentas, opaque oranges, subtle umbers . . . and hundreds more.

Furthermore, the craftsmanship is spectacular. Lippman's painted designs for the stair towers were flowing abstractions with geological and cosmological overtones--the whole work is called "Dawn and Dusk." Working under contract to a 150-year-old German company--Franz Mayer of Munich--the artisans of Mosaici Artistici of Spilimbergo, Italy, translated Lippman's loose brushwork into the hard medium of mosaic in a remarkably freehanded spirit.

Lippman and Van Dusen won a competition to do the artwork. Their strategy was exceedingly smart. There was, of course, a limited budget, and the pair's idea of concentrating attention on the four stair towers--the most sculptural elements of a strongly sculptural building--was both cost-effective and responsive to the architecture.

The only major shortcoming in this marriage of art and architecture is that there simply isn't enough of the art. The idea of converting the towers into giant stalks of glimmering color was splendid, but the budget ran out at about 2,000-square-feet worth of mosaic panels--quite something, but not enough.

Part of the problem here is that the artists came so late in the game--the building, strongly designed by Skidmore, Owings & Merrill and completed in 1986, originally was surrounded by metal trellises for climbing plants. When the trellises needed replacement, officials decided to spend the maintenance money--$280,000--on the art.

On the other hand, a really good idea partially fulfilled is a lot better than a bad idea spread all over the place, which often happens with large public commissions. And this was a really, really good idea. These wonderful mosaics have, I am confident, brightened up many a day in New Carrollton.

CAPTION: Triangles of glass and stone, randomly placed on the Metro plaza clock tower and columns, add sparkle to morning and evening commutes.