SOUTH SALT LAKE — Just before the TRAX trains whiz past, the wires crinkle and buzz above. That's a warning to Kim Martinez to raise her hand to the train conductor as an "I see you, you see me" greeting.

Martinez waves for a few seconds, then turns back to the task at hand — a 2,500-square-foot mural covering the side of a construction supplies building in South Salt Lake. She and her dozen art students from the University of Utah have been painting the wall faithfully — rain, heat or cold — since September and hope to finish within the next three weeks.

The mural, on the east side of Standard Builders Supplies at roughly 2700 South along the TRAX lines, shows a mass of railroad tracks winding between two large workers, who join the rails at the center of the mural as a symbol of the Golden Spike — the name of the mural. The ubiquitous Wasatch mountains, Great Salt Lake, and Salt Lake City are in the background. The design harks to the Roper rail yards in South Salt Lake and the city's blue-collar tradition.

But, more than the art, the mural is about the community's involvement. More than a dozen people have spent weeks creating the mural, and thousands see it each day as their trains rush past.

"The only way you can see these is going 50 miles per hour," Martinez said. "You have to be on TRAX to see this. Who rides this train? It's everyone from Salt Lake City to Sandy."

The painting is exhausting for the students. They spend about seven hours each Friday on the scaffolding. In September's heat, they baked in the reflection off the still white, unpainted wall. During October weekend storms, they wore trash bags and tried to prevent the water-based paint from running. Now in late November, they're bundled up in layers and trying to keep paint-covered fingers from freezing.

"The weather makes it a lot harder, and it's exhausting," student Carlos Perez said. "But I'm enjoying being a part of it."

In two previous mural-painting classes, Martinez allowed the students to choose their own murals. This time, however, she wanted to challenge them with her own creation, so she did the model painting over the summer. When the class found a site for the painting, she stretched her design to the building's dimensions. After the class applied 20 gallons of thick primer, the students painted a grid on the blank wall with 1-foot squares. Next, they drew on Martinez's design with chalk lines, and firmed up the chalk lines with spray paint. They followed up with a base coat of violet and then the outer layers of paint.

Martinez is teaching her students a glaze painting technique, applying thin layers of paint over base colors that will add depth to the final product. The result of the glaze is a brown skin that is really five different shades of browns, blues and violet. Corners of the mural still have the vibrant violet showing. Eventually, a student in a boom truck will paint over it to match the cerulean sky and wispy clouds.