The Brain of the Train: Understanding the On-Board Unit (OBU)

In modern signaling systems like ETCS (European Train Control System), the safety logic has moved from the trackside to the train itself. The OBU (On-Board Unit) is the complex assembly of hardware and software responsible for this task. It acts as the train’s “brain,” making safety-critical decisions in milliseconds.

Key Components of an OBU

The OBU is not just a single box; it is a network of devices connected throughout the locomotive:

• EVC (European Vital Computer): The core processor. It takes all inputs, performs the safety calculations, and decides whether to allow the train to proceed or to apply the brakes.
• DMI (Driver Machine Interface): The touch-screen display in the cab where the driver sees the speed, target distance, and system status.
• Odometry Sensors: Radars and wheel sensors that calculate exactly how fast the train is moving and how far it has traveled.
• Antennas: To communicate with Eurobalises (under the train) and GSM-R/Radio networks (on the roof).

How It Works: The “Braking Curve”

The OBU constantly calculates a “Braking Curve.” Imagine a dynamic graph that shows exactly when the train must start braking to stop safely at a red signal ahead.

It compares the train’s Actual Speed (from odometry) with the Permitted Speed (calculated by the EVC). If the actual speed crosses the safety curve, the OBU cuts the traction power and applies the emergency brakes automatically.