The ERTMS system is divided into 2 subsystems: onboard and trackside, as shown in the figure below.
The ERTMS components corresponding to the ERTMS subsystem installed on the trackside are:
Eurobalise
The Eurobalise is a passive device that is installed on the track, storing data (fixed or switchable, i.e. with the possibility of changing information content) related to the infrastructure, such as speed limits, position references, gradients, etc. It is a passive device because it does not need an electric supply since it is the train antenna (BTM) that energises it when passing over it.
LEU (Lineside Electronic Unit)
The LEU is the interface between the Eurobalise and interlocking. The LEU acquires the information from the interlocking, and in concordance with the lineside signalling (if available) sends the appropriate information (called a 'telegram') to the Eurobalises, which in turn send it to the onboard system.
Euroloop
The Euroloop is an optional component, applicable to ETCS Level 1 to increase system performance. The Euroloop provides signalling information to the ETCS onboard system in advance of the next Eurobalise in the train running direction.
RIU (Radio Infill Unit)
The RIU is an optional component, applicable to ETCS Level 1 to increase system performance. It allows the message contained in a Eurobalise to be sent in advance to the train by GSM-R transmission. Thus, a train located in the coverage area of the radio infill can constantly receive updated information, e.g. if the route is established and the train is authorised to proceed, this information will be received by the onboard system without the train needing to read the balise located at the signal.
Radio Block Centre (RBC)
The RBC is a device used at ETCS Level 2 and Level 3 acting as a centralised safety unit, which, using radio connection via GSM-R, receives train position information and sends movement authority and further information required by the train for its movement. The RBC interacts with the interlocking to obtain signalling-related information, route status, etc. It is also able to manage the transmission of selected trackside data and communicate with adjacent RBCs.
Interlocking
Interlocking is not an ERTMS component, but it plays an essential role in the signalling system. Interlocking guarantees safety for train movements or routes and ensures that the route for a specific train is established and that incompatible routes are not simultaneously established. The interface between the interlocking and the ERTMS trackside subsystem is necessary in almost all ERTMS structures.
Control Centre
The Control Centre is the signal box that covers all the routes and trains running in a specific area. A Control Centre usually commands several interlockings, comprising one or several railway lines.
Among onboard components, the following can be mentioned:
Euro Vital Computer (EVC)
The EVC is the core of the ETCS onboard device. It is part of the Automatic Train Protection logic and is the unit with which all the other train functions interact, such as the odometer or the GSM-R data reception.
Driver Machine Interface (DMI)
The DMI is the interface between the driver and the ETCS. In most cases, it is an LCD touch screen panel for control and indication functions, allowing the driver to enter the required input data into the system and to visualise the output data.
Train Interface Unit (TIU)
The TIU is the interface that allows the ETCS to exchange information (e.g. ETCS will receive the status of the direction controller in the cab: forward, neutral or backward, which represents the direction of the train movement, through the TIU) and issue commands to the rolling stock (e.g. ETCS will send the rolling stock the command to apply the brakes).
Juridical Recording Unit (JRU)
The Juridical Recording Unit provides ‘black box’ functions, i.e. it stores the most important data and variables from train journeys, allowing later analysis.
Balise Transmission Module (BTM)
The BTM is a module inside the ETCS onboard equipment for intermittent transmission from track to train, which processes signals received from the onboard antenna and retrieves data messages from a Eurobalise.
Loop Transmission Module (LTM)
The LTM is an optional module inside the ETCS onboard equipment for transmission from track and train, which processes signals received from the onboard antenna and retrieves data messages from a Euroloop.
Onboard radio communication system (Euroradio)
The GSM-R onboard radio system is used for the bi-directional exchange of messages between the onboard subsystem and RBC or radio infill unit.
The onboard Euroradio is a module inside the ETCS onboard equipment for exchanging messages between track and train, which processes signals received from the onboard antenna and retrieves application data messages from the RBC or the RIU.
Odometry system
The odometer is responsible for calculating the distance run by the train, typically consisting of redundant tachometry and radar, able to calculate distance, speed and acceleration.
Specific Transmission Module (STM)
The STM is a device that allows the ETCS onboard equipment to be interfaced with the onboard part of an existing National Train Control system. It will enable smooth transitions from/to the national system(s) and gives access to some ETCS onboard resources (e.g. DMI).
Depending on its functionality and the desired configuration, the national system can be addressed either via an STM using the standard interface or via another national solution.