What is the LZB system and what is it for?

The LZB system is one of the safety mechanisms that monitor the circulation of high-speed trains. The failure of the system is one of the axes of the investigation into the rail accident that occurred this Wednesday in the vicinity of Urda (Toledo). A AVE train from Renfe crashed into an Iryo train on the Madrid-Andalucía high-speed line.

But what is the LZB system and how does it work? The LZB is the German word Linienzugbeeinflussung . It's an automatic train control and protection system that continuously monitors the running of trains and helps maintain safety distances between them.

How does the LZB system work?

The system transmits information to the driver through a conductive cable along the track, which controls the speed and traffic conditions of the train and verifies that the gear is within the permitted limits.

If it detects that the train is exceeding the permitted speed, it can activate the automatic brake, thus helping to avoid danger situations and reducing the need for the driver to depend on the signals he sees from the cabin.

This supervision is particularly important on high-speed railways, because trains are running very fast, and they need a lot of distance to stop completely.

What happens if the system stops working?

When the LZB system fails, trains can be automatically stopped for safety. The resumption of traffic must be carried out in accordance with the procedures established for this type of situation, ensuring that each train receives the necessary permits and instructions.

In these circumstances, routine automatic supervision is not available, so security is more dependent on side signals, communications with the control centre and instructions given to train drivers, and traffic must be adapted to the conditions of safe transit without conventional systems.

With regard to the Urda accident, according to the first reports, the LZB system failed and several trains were stopped. The investigation must clarify how traffic was managed afterwards and what instructions were received by the AVE that crashed into the Iryo train.

They shall also establish whether the safety protocols provided for in these cases were correctly complied with and the impact of the technical failure on the accident.