Dallas transit planners are debating how much more subway transit the city should have as suburban communities seek the same construction money for a Cotton Belt rail line.

Downtown Dallas supporters want the so-called "D-2" DART rail downtown expansion project to be entirely underground to reduce traffic and neighborhood conflicts up above.

"A subway has the unique capability of moving people because it doesn't interfere with the surface traffic," said Dallas City Councilman Philip Kingston.

Dallas already has a three-mile subway segment between downtown and Mockingbird Lane that opened in 1997. The single underground station beneath the City Place high rise is 120 feet beneath the Central Expressway. Boring equipment was used to make the tunnel with little disturbance above.

DART

"I think it's actually much simpler than it used to be," Kingston said. "I think tunneling technology is a lot more efficient than it used to be and cheaper."

A $1.57 billion subway extension boring project is currently underway in San Francisco. DART estimates making the entire D-2 project a subway could cost $1.3 billion. DART is currently reviewing several options to juggle money with the Cotton Belt line, proposed between Plano and Dallas/Fort Worth International Airport, to get both projects completed at the same time.

Boring a subway tunnel beneath older downtown Dallas buildings would present challenges, according to civil engineer Tom Witherspoon. He has experience with downtown Dallas excavation work.

Witherspoon said some older buildings have no accurate plans to verify the depth of their foundations.

"Those boring machines are going to bore through solid rock. There will be Austin chalk, limestone and grayish shale, and you have to be careful you don't hit the piers for that building," Witherspoon said. "There's enormous liability with that. Also because we're close to the Trinity River, there's potential water problems. All of that has to be mapped out before they do anything."

DART board members reviewed five alternatives for splitting money between D-2 and Cotton Belt in a 20-year financial plan at a meeting Tuesday evening. One option includes reducing the Cotton Belt from D/FW Airport to only Addison, with an extension to Plano later. Addison and Plano leaders oppose that option. Another option is putting half the D-2 project at street level.

Kingston said subways have been successfully constructed in other older urban areas and the extra expense of an all subway D-2 project is justified for downtown Dallas.

"As we get more people used to using DART, we're going to have much more demand, and that's what we really want. That's what's going to take cars off the street. That's what's going to support the density of development that we see here," Kingston said.