Advanced Civil Speed Enforcement System (ACSES) is a positive train control cab signaling system developed by Alstom. The system is designed to prevent train-to-train collisions, protect against overspeed, and protect work crews with temporary speed restrictions. The information about permanent and temporary speed restrictions is transmitted to the train by transponders (Balises) lying in the track, coded track circuits and digital radio. It was installed beginning in 2000 on all of Amtrak's Northeast Corridor (except MTA territory) between Washington and Boston, and has been fully active since December 2015, a few months after the 2015 Philadelphia train derailment which it would have prevented.
