The fundamental rule of railway safety is simple: Keep trains apart to prevent collisions. However, how engineers achieve this separation determines the capacity of the entire line.
The industry is currently undergoing a massive shift from traditional Fixed Block systems (hardware-based) to modern Moving Block systems (software/communications-based). Understanding this difference is key to understanding technologies like CBTC and ETCS Level 3.
🧱 What is a Fixed Block System? (The Traditional Way)
Since the 19th century, railways have been divided into physical sections called “Blocks.” These are defined by specific start and end points, usually marked by a signal or an insulated rail joint.
- The Rule: Only one train is allowed in a block at a time.
- The Tech: It relies on Track Circuits or Axle Counters to detect the train’s presence.
- The Problem: Even if a train is at the far end of a 2 km long block, no other train can enter that block. This creates large “empty spaces” on the track, severely limiting how many trains can run per hour (Headway).
📡 What is a Moving Block System? (The Modern Way)
Moving Block removes the physical divisions. Instead of fixed sections on the ground, the “block” becomes a digital “Safety Bubble” that surrounds the train and moves with it.
- The Rule: A safe braking distance is calculated in real-time based on the train’s current speed and weight.
- The Tech: It uses radio communication (GSM-R or WiFi) between the train and the control center. The train constantly reports its exact position.
- The Advantage: As the train slows down, the safety bubble shrinks. This allows the following train to get much closer safely, significantly increasing the line’s capacity.
⚔️ Comparison: Fixed Block vs. Moving Block
| Feature | Fixed Block System | Moving Block System |
|---|---|---|
| Train Separation | Physical track sections (static). | Dynamic “Safe Zone” around the train. |
| Train Detection | Track Circuits / Axle Counters. | Radio / Transponders / GPS. |
| Capacity (Headway) | Low to Medium (2-5 mins). | High (Can go below 90 seconds). |
| Trackside Equipment | Heavy (Cables, Signals, Lights). | Light (Markers, Balises). |
| Examples | Traditional Mainlines, ETCS L1. | Modern Metros (CBTC), ETCS L3. |
💡 Why Moving Block is the Future?
Moving block systems allow for “Target Distance Braking.” Unlike fixed block systems where a driver (or computer) brakes in “steps” as they pass yellow/red signals, moving block allows for a smooth braking curve directly to the stopping point of the train ahead.
This technology is the backbone of driverless metros (ATO systems) because computers can calculate these safety margins much faster and more precisely than humans.
❓ FAQ: Moving Block Technology
Is Moving Block safer than Fixed Block?
Both systems are extremely safe when implemented correctly. However, Moving Block offers higher precision. It knows the train’s speed and location to the centimeter, whereas Fixed Block only knows which “section” the train is in.
Why don’t all railways use Moving Block?
Cost and complexity. Moving Block requires all trains to be equipped with expensive computers and radio systems. For low-traffic rural lines, the traditional Fixed Block system is much cheaper and sufficient.
Does ERTMS use Moving Block?
It depends on the level. ETCS Level 1 and Level 2 are typically superimposed on Fixed Block sections. Only ETCS Level 3 is a true Moving Block system that eliminates physical track detection devices.
