Metro's promised upgrade to cell phone service in its underground rail system is behind schedule.

The transit agency had pledged to have all of its 47 underground rail stations fully wired by last October. But so far, only the 20 busiest underground stations provide steady signals to all riders.

"We are not in a position to know when it will be done," Metro spokesman Dan Stessel told The Washington Examiner. He said the cell phone carriers who are paying Metro more than $50 million over 25 years are in charge of adding the necessary upgrades to let riders talk uninterrupted throughout their commutes.

Metro has had cell phone service in its rail system for about 18 years -- but only readily available for Verizon customers and spotty even then. As part of a deal with Congress, the agency is required to upgrade the system to provide service for all major cell phone carriers in exchange for $1.5 billion in federal funding over 10 years.

The legislation set deadlines. By October 2009, Metro had to have service in the 20 busiest underground stations. By October 2012, it is supposed to have the entire system wired and active.

However, the agency also offered its own deadline, promising to have the remaining 27 underground stations up and running by last fall. Those stations still don't have cell phone service.

Work in the underground stations is 80 percent done, Stessel said, and crews have finished installing about 75 percent of the underground fibers in the tunnels that connect the stations. That means some riders may notice that their service is stronger in areas that already had the wiring.

But the service is somewhat like an old-fashioned string of Christmas lights that cannot be turned on at new stations unless most of the components are in place.

Stessel said Metro would prefer an incremental approach of activating the service station by station as it becomes ready. "We understand that's not always possible given the way the system is designed," he said.

The challenge is that the cell phone companies need access to the tunnels to install the fibers and hardware to carry the signals. To do that, they need to have trained Metro workers accompany them, but the number of agency workers with that level of safety training is limited, Stessel said.

"The work takes priority over everything else other than safety-related track work," he said.

Meanwhile, next year's congressionally mandated deadline to have service systemwide by October 2012 looms. "We're working toward that goal," Stessel said.