For a railway signaling system to be safe, it must answer one fundamental question: “Is there a train on this section of track?”
Traditionally, this was done using Track Circuits. However, modern railways are increasingly switching to a more robust alternative known as the Axle Counter. This system acts like a digital accountant, ensuring that every wheel that enters a section also leaves it.
🧮 How Does an Axle Counter Work?
The principle of an axle counter is based on simple arithmetic. The railway line is divided into sections, with a “Counting Head” (sensor) installed at the beginning and end of each section.
The logic follows a three-step process:
- Count In: As a train enters the section, the first sensor detects the magnetic disturbance of the passing wheels and counts the axles (e.g., +40 axles).
- Store: The system marks the section as “Occupied” (Red Signal) because the number of axles inside is greater than zero.
- Count Out: As the train leaves the section, the second sensor counts the exiting axles. If the “Count Out” equals the “Count In” (40 – 40 = 0), the system confirms the section is clear and turns the signal Green.
⚙️ The Technology: Magnetic Sensors
Unlike track circuits, axle counters do not require the rail itself to conduct electricity. Instead, they use magnetic field sensors bolted to the web of the rail.
- Two coils are used in each sensor box to detect both the presence and the direction of the train.
- Because they don’t rely on electrical contact, they work perfectly even on rusty rails or in flooded tunnels where track circuits would fail (short circuit).
⚔️ Axle Counters vs. Track Circuits
This is the biggest debate in railway infrastructure engineering. Each has distinct pros and cons.
| Feature | Axle Counter | Track Circuit |
|---|---|---|
| Detection Method | Magnetic sensors (Point-based). | Electrical current in rails (Continuous). |
| Broken Rail Detection | No. (Cannot see breaks). | Yes. (Current stops flowing). |
| Weather Resistance | High. Works in flood/wet conditions. | Low. Water causes short circuits. |
| Cable Length | Can transmit data over long distances via fiber. | Limited length due to resistance. |
| Maintenance | Low (Solid state electronics). | High (Insulated joints wear out). |
🚧 The “Ghost Train” Problem
The main disadvantage of axle counters is the reset procedure. If a power failure occurs or a maintenance trolley is lifted off the track mid-section, the counts will not match (e.g., 40 in, 0 out).
The system will assume a train is still there and keep the signal Red indefinitely. This requires a manual or cooperative “Reset Procedure” by the signaler to confirm the track is actually empty, which can cause operational delays.
❓ FAQ: Axle Counter Technology
Can axle counters detect broken rails?
No. This is the biggest drawback. Since axle counters only look at the start and end points, they cannot detect if a piece of rail has snapped in the middle of the section. Track circuits are superior in this specific safety aspect.
Do axle counters work on steel bridges?
Yes. On steel bridges, insulating the rails for track circuits is extremely difficult and expensive. Axle counters are the standard solution for bridges because they don’t require insulation from the structure.
What happens if a sensor misses a wheel?
The system is designed to be “Fail-Safe.” If the exit count is lower than the entry count (e.g., missed detection), the system assumes a part of the train (a wagon) has detached and been left behind. It will hold the signal at Red to prevent a collision.
