Metro has taken dozens of 4000-series railcars off its tracks because of a potential safety concern involving the trains' automatic control system, Metro announced Thursday.
Metro General Manager and CEO Paul Wiedefeld ordered the immediate removal of the trains from service.
"Metro railcar engineers believe the potential exists for an undetectable failure of the 4000-series [automatic train control] ATC system control board that could result in improper speed commands being given to a train when a 4000-series car is in the lead position," Metro said in a news release.
Metro had all 4000-series cars removed from its mainline tracks by about 5:20 p.m. The 82 railcars make up 7 percent of Metro's 1,212 fleet, Metro said.
Metro riders may notice fewer eight-car trains in service over the next several days.
"Today's action is being taken in an abundance of caution and, while we believe that the risk is small, it is a risk I am unwilling to take," Wiedefeld said in the release. "Everything we do here is going to put safety first, no matter what."
The 4000-series railcar manufacturer recommends annual testing as a way to mitigate the risk of a false indication, but such testing is not currently done at Metro, the transit agency said.
Metro may consider "bellying" 4000-series cars in the center of trains at a future date. The ATC issue identified is not a risk when the 4000-series cars are not in the lead position, Metro said.
The 4000-series is the smallest and least reliable of Metro's six "legacy" fleets. There are 41 married pairs of 4000-series cars currently in active service, and Metro was already considering retiring all of them by the end of 2017. Metro said it may retire the trains sooner in light of the issue.