DEV Community: George Tomzaridis

Inside Thessaloniki Metro: The Tech Behind the Driverless System

George Tomzaridis — Sat, 15 Feb 2025 01:02:31 +0000

I've been a train enthusiast since I was a kid. If I hadn't chosen a career in IT, I'd probably be a train driver or working somewhere in the railway industry. As I ventured into IT and eventually focused on telematics and the Internet of Things (IoT) in 2022, my passion for trains never faded. Instead, I became increasingly interested in understanding the technology behind modern railway systems and how I could merge my expertise in IT, with my love for trains.

I first started studying railway systems by learning the fundamentals of signaling, safety, and operations. Over time, I deepened my knowledge and eventually earned four certifications covering various aspects of railway technology. This journey not only enhanced my understanding of how rail systems function but also allowed me to explore the intersection of IT and the railway industry. Along the way, I also gained insights into important safety standards like the European Train Control System (ETCS), Centralized Traffic Control (CTC), and Driverless Technology (Grades of Automation - GoA).

In this article, after many hours of studying, practice, and researching we will take a deep dive into the Thessaloniki Metro’s driverless train system, exploring the technology that powers it, from signaling and automation to communication networks and control systems.

History behind Thessaloniki Metro

Let's go for a short history lesson. The Thessaloniki Metro, a long-awaited project for Greece's second-largest city, has been a significant milestone in the country's urban development and transportation evolution. The idea of building a metro system in Thessaloniki dates back to the early 1980s when the city's rapid population growth and traffic congestion began to demand innovative public transport solutions. However, it wasn't until the late 1990s that serious efforts were made to bring the project to life. Construction officially began in 2006 and after a series of delays, technical challenges, and archaeological discoveries along the route,
the first major line, Line 1, opened on November 30, 2024, marking a major milestone in the city's public transport development. Line 1 consists of 13 stations, with the entire route running over 9 kilometers, connecting the New Railway Station to Nea Elvetia.

Basic Railway Terminology

To understand and deep dive into the whole infrastructure, we need to first understand some basic terms, that are very common in the railway industry, by looking at the following network map designed by me and then analyze each component alone.

In the above map analysis, we see the following components:

Tracks: Fundamental infrastructure on which trains run. Thessaloniki Metro uses a modern alternative where rails are mounted on concrete slabs instead of ballast, often used in metro systems for durability and lower maintenance.

Crossovers (scissors crossover): A crossover is a section of track that connects two parallel tracks, allowing trains to switch between them. Thessaloniki Metro has a total of 4 crossovers, between specific stations:

New Railway Station X Dimokratias
Sintrivani X Panepistimio
Analipsi X 25th Martiou (we are not counting the single crossover for each track of the future expansion/connection with Line 2 toward Kalamaria)
Voulgari X Nea Elvetia

Depot & Train Yards: The depot is where trains are stored, maintained, and serviced. Specifically in the terminal station "New Railway Station" trains go to hidden Stabling tracks, where are parked when not in operation.

Train Yard / Temp Depot in New Railway Station

The trains temporarily park here to start sooner from the other side of the line route and for track-switching purposes.

Main Depot after Nea Elvetia terminal station

Here the trains park when they finish all their routes and workers do inspections, repairs, and overhauls.

Turnouts & Switches: A turnout (also called a switch or points) is a mechanical assembly that allows trains to move from one track to another.

Interlocking System: A crucial safety mechanism in railway operations that prevents conflicting train movements by ensuring that signals and track switches (turnouts) are set correctly before a train is allowed to proceed. It is designed to eliminate human error and ensure trains do not collide or take incorrect routes.

Aspects & Signaling: Signaling is the system that controls train movements, preventing collisions and ensuring efficient operation. Aspects refer to the different signal indications given to train drivers (or automated systems). Metro Thessalonikis uses "Moving Block Signaling" which can transmit real-time train position data to dynamically adjust safe distances between trains. The protocol standard is called CBTC (Communications-Based Train Control).

Third Rail / Power System: Railway systems need a power source for electric trains. A metal rail placed alongside or between the running rails supplied 750 VDC power via a shoe contact on the train.

Third Rail in the corner

How the train touches the Third Rail and gets power (under the third rail for safety reasons and to minimize the risk of electrocution)

Power Distribution

You may be guessing that the system has a continuous "Third Rail" across all the line networks. The answer is no, and the reason may surprise you! But now you are wondering... how the power is disturbing across the whole system, with gaps and no continuous connection.

The whole power distribution system is grouped in substations in each station with special electrical equipment. With this approach, we can balance the load in the entire network and reduce the risk of short circuits or blackouts. Also, this helps us to fix any potential problems quicker on a track, without stopping the service or affecting the train schedule, by cutting the power to a specific area only.

The operators in the Control Center and the station manager, continuously monitor the power infrastructure using SCADA (Supervisory Control and Data Acquisition), which alerts for any problems that occur and also inter-connects with other systems to make the train journey safer and quicker (will see how later).

SCADA system in the Operations Center

Driverless Trains

Finally.. let's talk about the trains and the whole structure of the systems included inside to keep us safe and happy. The Thessaloniki Metro operates with Hitachi Rail Italy Driverless Metro trains, designed specifically for fully automated, driverless operation under the Grade of Automation 4 (GoA4) standard. These modern trains are lightweight, energy-efficient, and optimized for safe and reliable operation without onboard drivers.

All the trains rely on an advanced CBTC (Communication-Based Train Control) system to ensure smooth and safe operation. This system integrates multiple automation layers, including ATO (Automatic Train Operation) & ATP (Automatic Train Protection).

CBTC (Communication-Based Train Control)

CBTC (Communication-Based Train Control) is an advanced railway signaling system that enables high-capacity, driverless train operations by continuously monitoring and controlling train movements in real time. It replaces traditional fixed-block signaling with a more dynamic and precise moving block system, allowing trains to run closer together while maintaining safety.

Each train has a CBTC computer that determines its exact position, speed, and operational status. Trackside sensors, antennas, and beacons provide redundancy and additional location data.

ATO (Automatic Train Operator)

Automatic Train Operation (ATO) is a railway system that automates the movement of trains, reducing or eliminating the need for human intervention in driving (aka Super Computer / Brain of the train). It works alongside signaling systems like ATP (Automatic Train Protection) to ensure safe and efficient operations. The main tasks of this system are:

Acceleration & Braking
Station Stopping
Door Operation
Voice Announcements inside the train
CCTV Cameras inside the train
Control the PID (Passengers Information Displays) on board.

Inside the train

Outside the train

Mainly the train is in "auto" mode (GoA4 - Fully Driverless/Unattended Train Operation), but in emergencies and specific cases, staff can come and control the train manually via a central console.

ATP (Automatic Train Protection)

Automatic Train Protection (ATP) is a railway safety system that prevents collisions, derailments, and speeding by automatically enforcing speed limits and stopping the train if necessary. It works alongside other systems like ATO (Automatic Train Operation) and ATS (Automatic Train Supervision) to ensure safe and efficient train movement:

Constantly checks the train’s speed against predefined limits and automatically reduces speed or applies emergency braking.
Ensures that the train follows signal instructions and if passed a red signal, the train stops immediately.
Calculates braking distances based on train speed, track conditions, and obstacles ahead. The result is to stop safely before and danger point or at a specific distance (on a station aligned with the platform doors).
Interconnect with PCS (Passenger Communication System) to support emergency telephones/buttons inside the train.

Control Center - Centralized Traffic Control

The Control Center, or CTC (Centralized Traffic Control), is the nerve center of the railway or metro operations. It serves as the central point for monitoring and controlling the entire rail network. The CTC is primarily responsible for ensuring the safe and efficient movement of trains throughout the system, coordinating traffic, and handling emergencies. The main tasks handled by the CTC are:

Traffic Management
Signalling
Routes / Schedules

Do you remember some rows above that I mentioned that the SCADA interconnects with other systems? It's time to find the big WHY and HOW.

What if we have a power outage? How the trains will react?

Oops. Huston, we have a problem. A short circuit knocked down the power on one substation and some part of the network is "dead". Now what?
Can trains pass the "dead" zone? Actually yes and no. But why?
First of all the ***SCADA* will detect the power problem*, issue a warning to the operators, and transmit this information to other systems. One system is the **CTC which will notify all the trains* (specifically the ATO) about the incident, by changing the signaling state (the aspect lights will turn red). This allows the ATO computer (by the way the ATO has power even in this case due to some backup batteries inside the train that keep the safety and critical systems operational) of each train to break smoothly before passing the "dead" zone and wait there for further instructions about the routing scenario that will follow from now on.
Also, the ATP will prevent any unauthorized access or passing the red signal for any train, to ensure safety and flexible rerouting.
If some trains are inside already the "dead" zone will just slow down due to a power outage (maybe with a little bit of luck they go to the next zone that has power and continue their journey), move a couple of meters due to physics, and then stop (normally or with the help of the brakes).
If the outage is serious and takes time to go back online, the operation center will cut the power (for the safety of the passengers) in a larger group of the line and staff will help the passenger evacuate the train safely, walking to the next station through the emergency walkway in the tunnel.

Example Scenario:

We have a power outage between Stations B & C in Track 2. CTC Computer & staff from the operator center discovered the problem from SCADA alerts and did a rerouting / changing signaling. Train A wants to go to Station B and needs to wait before switching tracks because Train B is already moving with a "caution" signal to Station D as normal. Train C can continue normally (doing the opposite direction, as a shuttle train, back and forward to move passengers much quicker) and use Track 1 to reach Station D on Track 2 because knows that no trains will come from the opposite direction (normally the Train A) due to a track problem.

Hooray, we saved the day and all the operations continue normally without any issues!

Network & Communications Infrastructure

We saw all the systems working under the hood, but how they connect and "talk" to each other. Thessaloniki Metro utilizes a private 5G network to facilitate ultra-fast, low-latency, and secure communication between the trains, wayside equipment, and the Centralized Traffic Control (CTC).

The private 5G system is isolated from public networks, ensuring uninterrupted and secure train operations. The 5G network utilizes high bandwidth and allows even video or voice data exchange at high speeds (used in emergency train announcements / giving commands to ATO computer).

Also, the usage of another protocol called TETRA (Terrestrial Trunked Radio) or DMR (Digital Mobile Radio), is very critical and helps staff members and the control center communicate with each other, even underground and in tunnels.

A Passenger’s Journey

Let’s break down what happens step by step as a passenger boards a train at New Railway Station and travels to Nea Elvetia:

ATS (Automatic Train Supervision) updates PIDS (Passenger Information Display System) and announces: "Next train to Nea Elvetia arrives in 2 minutes." Real-time data from CBTC & ATS is sent over the 5G network & fiber optic backbone. Platform speakers & visual signs update automatically.
CBTC continuously tracks the train and adjusts speed for precise stopping. CTC & ATS updates PIDS and an automated announcement plays: "Train to Nea Elvetia is arriving in Track 1". Platform screen doors align and prepare to open.
ATO (Automatic Train Operation) ensures doors open only when fully stopped. Onboard CCTV streams live footage to the CTC from the train and the station (Centralized Traffic Control). ATO checks for obstacles indoors using sensors & platform cameras. After some time ATO announces "Doors closing" and closing the doors.
ATO accelerates the train smoothly as per the pre-defined speed curve. ATP (Automatic Train Protection) ensures safe braking & speed limits. Inside the train, the Next Station Display & Audio Announcements update dynamically via train telematics & ATS.
Arriving at Nea Elvetia, CBTC & ATO instructs the train to slow down for track switching. ATO communicates with PIDS and plays "We are approaching terminal station". ATS signals a track switch using the interlocking system. The train moves to the correct track for the return trip. ATP / ATS & CTC verifies that the train switched tracks successfully and reset the interlocking point for the tracks.
CBTC instructs the train to slow down for an accurate stop. SCADA monitors power supply & infrastructure. The train stops automatically, and doors open once a safety check is completed. ATO will announce "You have arrived at Nea Elvetia. This is the last stop. Please exit the train."

Emergency Cell Broadcast Systems: How Life-Saving Alerts Reach You in Seconds

George Tomzaridis — Wed, 21 Aug 2024 00:21:50 +0000

I spent my vacation in Athens from 10-18 August 2024. I had a great time there, went to many places and saw many friends. Unfortunately at the same time a large fire, strikes North-East Athens, so many people needed to evacuate their houses immediately, to survive and not be burned by the massive and quickly spreading fire. It was heartbreaking to see people running from their homes, unsure of what would happen next. Fortunately for them, they received an emergency message on their mobile phones, informing them about the nearby fire and what are the next steps to evacuate to a safer area. But... how this is possible? How we can use technology to save lives? How does this whole operation work?

I had the same questions and to be honest with everyone, also I had many technical questions about that subject, so I decided to analyze it and write an article about it. So let's get started step by step with some basic information first.

What is a Cellular Network?

Probably currently you have a smartphone... maybe you are using a smartphone to read this article. Imagine that some features on your smartphone, which you use daily, suddenly no longer exist. It will be completely useless to have one then (like holding a brick in your hands)... you will not be able to receive or make calls, receive sms, surf the internet wherever you go, use your favorite apps, etc. But why? The answer is that a smartphone relies of course on a network, for you to do basic things. To be more specific they need a cellular network to work.

A cellular network is a communication network where the last link is wireless. It is based on the division of a service area into small, overlapping regions known as cells. Each cell is connected to a wider network. This arrangement allows cellular devices, such as smartphones and tablets, to communicate with each other and with the Internet.

Each cell has its equipment and the whole cellular architecture has its own too. The basic structure of the cellular network, to proceed further and get a basic idea, is analyzed below:

Mobile User (User Equipment - UE): Mobile devices, or User Equipment (UE), are the starting point of any cellular network. These include smartphones, tablets, and other portable communication devices. The UE is equipped with a SIM card that identifies the user and their subscription details. These devices communicate wirelessly with the nearest cell tower, sending and receiving data in the form of radio waves.
Base Stations (BTS): Base Stations, commonly known as cell towers, are the central communication points within each cell. A Base Transceiver Station (BTS) is located at each tower, containing the equipment necessary to transmit and receive radio signals to and from the mobile devices within its coverage area. The BTS converts these radio signals into digital data that can be processed and routed through the network.
Base Station Controller (BSC): The Base Station Controller (BSC) manages the radio resources and controls multiple BTS units. It handles tasks such as handover management (when a mobile device moves from one cell to another), frequency allocation, and the establishment of communication channels. The BSC ensures that each device is connected to the appropriate BTS, optimizing the network's efficiency.
Mobile Switching Center (MSC): The Mobile Switching Center (MSC) is the core component responsible for routing voice calls, text messages, and data within the cellular network. It connects the BSCs to the wider telecommunications network, managing call setup, termination, and routing. The MSC also handles the billing and subscriber management functions.
Home Location Register (HLR) and Visitor Location Register (VLR):
The Home Location Register (HLR) is a central database that contains information about each subscriber's profile, including their services, billing information, and current location within the network. When a subscriber moves to a different cell, the Visitor Location Register (VLR) temporarily stores their information to facilitate local communication without constantly querying the HLR.

Understanding Signaling in Cellular Networks

Now that we understand the basic structure of cellular networks, let's talk about signaling. If we want to describe signaling in one sentence, this would be: It's critical and has an important role in the system and communications in general. Without that, the network will not work as expected or at all.

Signaling is responsible for managing, controlling, and coordinating communication between devices and network infrastructure. Unlike user data (voice or internet traffic), signaling refers to the control information exchanged to establish and maintain communication sessions.

Signaling encompasses all the non-user data communication that is necessary for the network to operate. This includes:

Call setup and teardown: Establishing and terminating voice or data connections.
Handover management: Transferring a call or data session from one cell to another as a user moves.
Authentication and security: Verifying the identity of users and securing communications.
Network resource management: Allocating and managing bandwidth, frequencies, and channels.
Roaming: Enabling users to move across different networks while maintaining service.
Emergency Messages: Establishing and broadcasting emergency alerts to mobile phones.

In modern networks, signaling typically uses separate channels from user data (voice, internet). Protocols vary by network type, so we will just mention some of them per network type, but will not deep dive and analyze them one by one:

2G: SS7, MAP, IS-41
3G: SS7/MAP, RANAP, SIP
4G: S1-AP, X2-AP, Diameter, GTP
5G: NGAP, HTTP/2, PFCP

How Emergency Cell Broadcast Works

Now that we understand some basics about cellular networks and learn some new terminologies, we can finally move on.

Emergency alerts are sent using a technology called Cell Broadcast (CB), which is different from SMS or internet-based messaging. Cell Broadcast allows messages to be sent to all phones within the range of specific cell towers. The technology is inherently location-based and can reach many users simultaneously without overloading the network.

When an emergency alert needs to be issued, authorities like government agencies or emergency services decide on the content and the geographic area that should receive the alert. The message is then sent to the cellular network operators, who broadcast the alert via their cell towers. These towers transmit the message to all phones connected to them within the designated area. Phones within the range of these cell towers automatically receive the alert, typically accompanied by a distinct sound or vibration.

The area to be notified is determined by the geographical coordinates and radius or by selecting specific cell towers covering the desired region (each cell tower has a unique ID - The mobile network operator maintains a database that includes detailed information about each cell tower). Authorities provide this information to the network operators, who then ensure that the broadcast is only sent through the towers covering that area. If we use only coordinates or polygons/radius the system will contact the Geographic Information System (GIS) of the network provider, which stores information about each cell tower location, to find which cell towers are covered by the marked area and need to get the message.

The systems that play a critical role in transmitting successfully emergency alerts are:

Cell Broadcast Center (CBC): This is the core component responsible for creating and managing the broadcast messages. The CBC receives the content of the emergency alert from authorized sources, such as government agencies, and determines which areas should receive the message.
Base Station Controller (BSC): These are responsible for controlling multiple cell towers or base stations. The CBC sends the broadcast message to the BSC/RNC, which in turn instructs the relevant cell towers to transmit the message.
Base Transceiver Station (BTS): These are the actual cell towers or radio nodes that transmit the message to mobile devices. The message is broadcast over the air interface to all devices within the range of the cell tower.

Steps for transmitting an Emergency Alert

The Cell Broadcast Center (CBC) creates the message, specifying the content, the language, and the geographical area (e.g., using cell IDs or geographical coordinates).
Cell Broadcast messages are structured into "pages," each of which can carry a maximum of 82 bytes of information. A single-cell broadcast message can be composed of one or more pages, depending on the content length.
The Cell Broadcast Center (CBC) sends the message to the Base Station Controller (BSC), which controls the specific cell towers that cover the target area.
The message is broadcast over the air to all connected devices of each cell tower.
Phones within range of the targeted cell towers receive the message automatically. These alerts are usually accompanied by a distinct sound or vibration. Since Cell Broadcast is transmitted over signaling channels (like the Paging Channel (PCH) or Control Channels (CCH)), even if the network is congested or the phone is not in use, the message will still be delivered and displayed to the user.

Cell Broadcast messages are carried over signaling channels, not the same channels that handle regular user data (voice, internet). This makes the delivery of emergency alerts reliable even under heavy network congestion. In 2G, for instance, CB messages use the Broadcast Control Channel (BCCH), while in 4G/5G, they utilize the Physical Downlink Control Channel (PDCCH).

Since signaling channels are always monitored by mobile phones—even when idle or in low signal areas—this ensures that alerts can be received in almost any condition, including in some underground or low-coverage areas.

Example of a Cell Broadcast Message (for reference only)

Message ID: 4371
Message Type: Emergency Alert
Page Count: 2
Language: English (en)
Geographic Scope: Polygon
Polygon Coordinates: [(37.7749, -122.4194), (37.8044, -122.2711), (37.6879, -122.4702)]
Cell IDs: [1024, 1025, 1031, 1035]
Expiration Time: 15 minutes
Priority: High

"EMERGENCY ALERT: Severe weather warning in your area. Seek shelter immediately and follow instructions from local authorities."

Message ID: A unique identifier for the broadcast message.
Message Type: Specifies the type of alert (e.g., Emergency Alert, Weather Warning).
Page Count: The total number of pages if the message exceeds the 82-byte limit for a single page.
Language: The language of the message content (e.g., en for English).
Geographic Scope: This defines the area as a polygon using a series of latitude and longitude pairs. For instance, the coordinates [(37.7749, -122.4194), (37.8044, -122.2711), (37.6879, -122.4702)] represent points that outline a region in San Francisco, California.
Cell IDs: A list of specific cell towers that should broadcast the message. This is usually determined by matching the geographic area (e.g., the polygon or coordinates) with the cell towers that cover that area.
Expiration Time: The time after which the message should no longer be broadcast.
Priority: Indicates the urgency of the message.

This message has 135 characters. Assuming 1 byte per character (for simplicity, as we're dealing with ASCII), this gives us 135 bytes of content. Since each page can carry 82 bytes, the message will be split into two pages.

Will transmit Page 1 first, followed by Page 2. These pages are sent through the Cell Broadcast Channel (CBCH). Your phone will receive Page 1, store it, and wait for Page 2. Once it has both pages, it reassembles them into the complete message and displays it on your screen.

What if I have No Signal / No Service? How I will receive the message?

When your phone displays "No Service," it typically means that your phone cannot establish a standard connection for voice or data services. However, this does not always mean that your phone is entirely disconnected from the network. In some cases, your phone might still be able to receive low-level signaling information, like emergency alerts, through the signaling channels. This could happen if there is enough connectivity for basic signaling but not enough for regular communication services. However, if your phone is completely out of the coverage area or in a dead zone (like deep underground without repeaters), it will not be able to connect to any signaling channels, and you will not receive any broadcast messages.

Why do I receive an emergency alert for a far-away / different emergency area from the area where I am currently located?

Sometimes we receive emergency alerts for areas that are not even close to us. Let's be honest, it can happen. If the government officials don't include our area in purpose in the emergency alert, this can happen due to a few reasons:

Cell Tower Coverage: Mobile phones can connect to cell towers that are relatively far away, especially in rural areas or places with fewer towers. If you're connected to a tower that is within the emergency broadcast area, you might receive the alert even if you're not in immediate danger.
Overlapping Coverage: Cell towers often have overlapping coverage areas. If you're near the edge of the broadcast zone, you might receive the message due to the overlap, even if you're slightly outside the intended area.
Network Propagation Effects: Sometimes, the broadcast might be received in unintended areas due to signal propagation conditions, such as reflections or atmospheric effects, causing the signal to travel farther than usual.

Conclusion

Cell Broadcast is a robust, efficient, and geographically targeted method of sending emergency messages. It operates across all cellular networks, including 2G, 3G, 4G, and 5G, and uses specialized channels to transmit small packets of data that are continuously monitored by mobile devices. These messages are sent with high priority to ensure they reach as many users as possible, even under challenging conditions (low bandwidth usage).

I want to thank publicly my friend Dimitrios Collier for giving me the idea to write a whole article about it, with all of our discussions / my research as we walked in the city center and just talking about it due to recent events with the wildfires in Attica.