SACEM (railway system)

The Système d'aide à la conduite, à l'exploitation et à la maintenance (SACEM) is an embedded, automatic speed train protection system for rapid transit railways. The name means "Driver Assistance, Operation, and Maintenance System".

It was developed in France by GEC-Alsthom, Matra (now part of Siemens Mobility) and CSEE (now part of Hitachi Rail STS) in the 1980s. It was first deployed on the RER A suburban railway in Paris in 1989.

Afterwards it was installed: In 2017 the SACEM system in Paris was enhanced with Automatic Train Operation (ATO) and was put in full operation at the end of 2018.
 * on the Santiago Metro in Santiago, Chile;
 * on some of the MTR lines in Hong Kong (Kwun Tong line, Tsuen Wan line, Island line, Tseung Kwan O line, Airport Express and Tung Chung line), all enhanced with ATO,
 * on Lines A, B and 8 of the Mexico City Metro lines in Mexico City; and
 * on Shanghai Metro Line 3.

The SACEM system in Paris is to be enhanced to a fully fledged CBTC system named NExTEO. First to be deployed on the newly-extended line RER E in 2024, it is proposed to replace signalling and control on all RER lines.

Operation
The SACEM system enables 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 slow speed, or 30 kph; 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. SACEM also allows for a reduction in potential train bunching and easier recovery from delays, therefore safely increasing operating frequencies as much as possible especially during rush hour.