Critical safety technology designed to automatically slow or stop trains that are going too fast was not working on a section of track outside of Seattle where an Amtrak train derailed, killing three people.

Sound Transit said Tuesday that the company was on schedule to have positive train control installed and operational in the spring, ahead of a December 2018 federal deadline.

The system would be able to take over control of a train when an engineer is distracted or incapacitated.

Sound Transit spokesman Geoff Patrick said the "vast majority" of equipment needed had been installed but not fully operational along the tracks and trains in the 14.5-mile (23.3-kilometer) section of line where the derailment occurred.