Metro officials have placed an indefinite hold on returning to computer-driven trains, a decision that comes more than two years after the agency vowed to revive the practice of automated operations.

The change in plans, first reported by WAMU, is part of continued concerns about the technology that was put on hold in the wake of the deadly 2009 Red Line train crash near Fort Totten.

The technology, known as automated train operation (ATO), uses software to determine the movement of trains throughout the system and control when, and how quickly, the trains accelerate and slow to a stop.

The technology is useful because it provides a smoother ride for passengers and helps prevent train bunching that can result in delays. But ATO is also a safety mechanism aimed at reducing the opportunities for human error on the rails. The issue of train operator inattentiveness has become more prominent in recent years, as the federal government has documented dozens of incidents in which operators have run red-light signals on the tracks, in some cases endangering passengers or colleagues.

In recent years, Metro has spent $106 million to install 1,730 new track circuits throughout the system — a project that was mandated by the National Transportation Safety Board, and was also performed in part to prepare for the eventual return to automated operations.

Metro briefly reintroduced ATO on the Red Line two years ago, allowing eight-car trains to be run in automatic mode during the morning and evening rush hours. Former general manager Richard Sarles had called that development “a milestone accomplishment,” and vowed to return all the subway lines to ATO by this year.

Now, Metro officials say they are not ready to permanently reintroduce the technology because they believe it is more important to focus on repairing track defects and updating the power system. They also are also concerned that reviving the technology now could jeopardize the safety of track workers and inspectors at a time when the agency is conducting more track reconstruction work during regular hours of operation.

“There are simply other priorities right now (e.g. SafeTrack),” Metro spokesman Dan Stessel said. “And not one dollar of the $106 million was wasted, because the track circuit modules needed to be replaced regardless of whether operating in manual or automatic.”

Stessel said Metro is not necessarily abandoning the automated technology, which was considered a premiere feature of the system when it debuted in 1976. The agency is planning to hire an expert signals consultant to determine whether ATO is still a good idea for Metro — and when, and how, to bring it back online.

“If the expert signals consultant determines that returning to ATO can be safely and efficiently implemented, [Metro] leadership will then determine the next steps,” Stessel said.

ATO is a common feature at transit systems around the world. But automated operations were halted in the Washington region in 2009, when a glitch in Metro’s ATO infrastructure was found to responsible for the Fort Totten crash. In that incident, the computer technology failed to detect a Red Line train that was stopped on the tracks. A second train, running on auto-pilot, slammed into the back of the first train at nearly 50 miles per hour. Nine people were killed.