Trains have been delayed on every Metra commuter line in the Chicago area during the Tuesday evening rush, due to a positive train control system outage.

Metra officials said, beginning around 5 p.m., trains on all of its commuter lines began experiencing positive train control issues, forcing them to stop. As of 6:15 p.m., Metra's positive train control system was slowly coming back online, but Metra said extensive delays were expected for many commuters as trains begin moving again.

Positive train control systems are designed to automatically stop trains if engineers fail to obey a signal or exceed established speed limits, in order to avoid train collisions and derailments.

The Federal Railroad Administration requires positive train control on all passenger and freight trains in the United States, and Metra's trains cannot operate without positive train controls in effect.

Metra officials said the cause of the positive train control outage was unknown Tuesday evening.