Metro seeks to bring back its beleaguered 7000-series trains this summer with the help of a new regular monitoring system — a suite of sensors in the tracks that can measure the distance between the wheels on each train car.

But the Washington Metrorail Safety Commission, which is providing oversight of the trains’ return, says this new system will need careful scrutiny.

In October, a train derailed outside the Arlington Cemetery when it moved through a switching area. No one was hurt, but investigators found the wheels on the train had moved too far apart — an issue that had sprouted up and been addressed nearly two dozen times in recent years.