The City Branch of the Reading Railroad runs more than two miles, from Broad Street near Callowhill to 27th Street and Pennsylvania Avenue, all below ground and most of it covered. Some exposed parts are overgrown with tall weeds.
"I definitely wanted to show the beauty of all the structures," Bruhin said, adding that he used light to bold effect in photos of the tunnel.
From 20th and Hamilton Streets to Pennsylvania Avenue, the 3,000-foot-long structure has a vented roof through which shafts of light illuminate parts of the dark interior.
The line dates back to the 1830s, when it was a ground-level freight line on the northern edge of a rapidly growing Philadelphia. By 1900, however, the Reading Co. had depressed the line, to get rid of some dangerous grade crossings.
The stretch, which had its tracks removed after The Inquirer and the Philadelphia Daily News stopped taking deliveries of massive rolls of newsprint at their former headquarters at Broad and Callowhill in 1992, when production was moved to the Schuylkill Printing Plant in Upper Merion, offers a below-the-surface look at a bygone era.
"It's dark but it's filled with color," Bruhin said of the tunnel. "The stone is beautiful and the structure is beautiful. It's the combination of dark and light together that really gets my attention."
The tunnel, parts defaced with graffiti, is unsafe for a variety of reasons, including debris.
In recent years there has been an effort to convert the City Branch and the Reading Viaduct, to which it connects near Broad, to a park. Part of it is inspired by the High Line, a park developed on an elevated former freight line in Manhattan.
Leah Murphy, president of the Friends of the Rail Park, a group advocating for a park on the stretch, said the first phase of developing the park, focusing on the viaduct, could begin as early as the end of this year. The organization said about $8 million has been raised for the project.
SEPTA, which purchased the stretch in 1992, also is looking to use it for high-speed transit, either bus or rail.
Byron Comati, the transit agency's director of strategic planning, said such a line could whisk passengers from the Art Museum to Broad. He said the possibilities are still being studied.
Bruhin said of his exhibit: "I definitely wanted to show the beauty in all the structures as much as I could. I wanted to bring out the light and the colors."
The free exhibit runs through March 31 at the High Point Cafe, 2831 W. Girard Ave.