The Système d'aide à la conduite, à l'exploitation et à la maintenance (SACEM) is an embedded, automatic speed control system for rapid transit railways. The name means "Driver Assistance, Operation, and Maintenance System".
Rapid transit systems that use SACEM include the RER A suburban railway in Paris; certain Santiago Metro lines in Santiago, Chile; and some of the MTR lines in Hong Kong (such as the Airport Express).
Operation
The SACEM system enable a train to receive signals from devices under the tracks. A receiver in the train cabin interprets the signal, and sends data to the console so the driver can see it. A light on the console indicates the speed control setting: an orange light means "Jog Mode", or 30 kilometres per hour (19 mph); a red light means full stop. If the driver alters the speed, a warning buzzer may sound. If the system determines that the speed might be unsafe, and the driver does not change it within a few seconds, SACEM engages the emergency brake.