Axle Counters
1. What is an Axle Counter?
- Definition: A track-side electronic device that counts the number of axles entering and leaving a signalling section (block section, siding, platform line, etc.).
- Purpose: To prove that the section is clear (no train or vehicle left behind) and thus allow the signal to be cleared for the next train.
- Philosophy:
“Count-in = Count-out ➔ Section Clear”
“Count-in ≠ Count-out ➔ Section Occupied”
2. Basic Building Blocks
- Track Device (TD) – magnetic sensor pair fixed to the rail web; detects metallic mass of wheel flange.
- Evaluator (EVAL) – outdoor/location box containing evaluation cards; converts analogue wheel pulses into digital “+1 / –1” counts.
- Transmission System – twisted-pair/FO cable to relay room.
- Axle Counter Unit (ACU) – indoor rack with counting/comparison logic; drives the track relay (Vm-ACR).
- Reset Unit – PRE (Pre-reset) and SR (Section Reset) buttons with seals/keys.
- Power Supply – 24 V DC ±20 %, ripple < 200 mV; battery back-up ≥ 8 h.
- Surge Protection – class-D, 10 kA 8/20 µs; rail potential < 500 V.
3. Working Principle Step-by-Step
| Stage | Event | Count | Section Status |
|---|---|---|---|
| 1 | First wheel hits TD-1 | +1 | Occupied |
| 2 | Train moves, wheels hit TD-1 | +1, +1 … | Occupied |
| 3 | Last wheel passes TD-2 | –1, –1 … | ? |
| 4 | Count register = 0 | 0 | CLEAR (ACR picks) |
| 5 | Register ≠ 0 | +/– n | OCCUPIED (ACR drops) |
- Minimum speed for reliable detection: 0.5 km/h
- Maximum speed: 250 km/h (standard) / 400 km/h (high-speed versions)
- Wheel sensor air-gap: 35–55 mm; can tolerate 30 mm vertical rail movement.
4. Key Specifications (IRS: S99/2007 & EN 15595)
| Parameter | Value |
|---|---|
| Operating temperature | –40 °C to +70 °C |
| IP-rating of TD | IP-68 |
| Count capacity | ±16 383 axles (15-bit) |
| False count probability | < 10⁻⁹ per axle |
| MTBF | ≥ 2.5 × 10⁵ h |
| Section length | 0.5 m to 25 km (with repeaters every 5 km) |
| Immunity to | traction return 750 A, 50 Hz; 100 A, 16⅔ Hz; 3 kV 1.2/50 µs surge |
| Safety integrity | SIL-4 (CENELEC), Fail-Safe (RDSO) |
5. Advantages over Track Circuits
- No need for insulated rail joints or impedance bonds – ideal for LWR track.
- Immune to rusty rail, poor ballast, flooding, oil, sand, snow, S&T bonding wire theft.
- Works on wooden, steel, concrete sleepers; any formation (earth, ballast, slab).
- One TD can be used for very short sections (10 m) – impossible with 50 Hz TC.
- Traction harmonics do not cause wrong-side failure.
6. Limitations / Drawbacks
- No broken-rail protection (TC still required in running lines as per Railway Board letter 2007/T-21).
- Count failure (drift) needs manual reset; causes delay.
- Initial cost 1.5× that of 50 Hz TC, but life-cycle cost is lower.
- Requires sealed reset – procedural safety; human error possible.
- Shunting movements/roll-back across TD in opposite direction can cause wrong count unless direction logic is provided (IRS: S99 recommends 4-sensor array for bi-directional).
7. Historical Milestones
| Year | Event |
|---|---|
| 1947 | First commercial axle counter (Däniker system) installed by Swiss Federal Railways. |
| 1952 | Deutsche Bundesbahn (DB) standardises “Spurkranz-Geber” (flange sensor). |
| 1975 | Integral evaluator with transistorised fail-safe AND gate (ABB). |
| 1984 | RDSO issues first Indian specification IRS: TC-41 (later revised to IRS: S99). |
| 1990 | Over 2 000 sections commissioned on Indian Railways (mainly sidings). |
| 1998 | Sil-4 certification achieved by Siemens AzS350U. |
| 2003 | Railway Board approves axle counters for running lines (with TC as complementary). |
| 2009 | First high-speed trial at 350 km/h on Spanish Madrid–Barcelona line (CSEE). |
| 2017 | RDSO approves dual-sensor (4-sensor) version for automatic restart (no PRE-reset) – called DACF (Drift-free Automatic Clearance Feature). |
| 2021 | Indian Railways floats global tender for 40 000 km track to be converted to RMSA (Route Relay Interlocking with Axle Counters) under “Signal & Telecom Works” budget ₹ 18 000 Cr. |
| 2023 | Gati-Shakti directive: all Green-field DFC and future HSR to adopt axle counters as primary block-proving equipment. |
8. Types / Variants in Indian Railways
- Single-section (point) type – Siemens AzS350U, Alstom EbiTrack 500, Kern micro-counter.
- Multi-section evaluator (MUX) – one outdoor evaluator caters up to 4/8 sections; reduces cable cost (used in Mumbai suburban).
- High-speed (HS) version – extended frequency response 2 kHz; certified 400 km/h (RDSO letter No. EL/3.1.2/5 dt 12.03.2019).
- Automatic_restart / DACF – uses second pair of sensors for self-correction; eliminates 90 % of right-side failures due to drift.
- Universal Axle Counter (UAC) – indigenous design by IRSEM (Railway Signalling & Telecommunication Engineering Module) – ₹ 3.5 lakh per section (50 % import substitution).
9. Installation Rules (SEM/AC/2018 & Signal Engineering Manual)
- Minimum distance between two TDs in same rail: 2 m (to avoid cross-talk).
- Distance from rail joint: ≥ 1 m; from S&T bond: ≥ 0.5 m.
- Height of TD top edge: 28 ±2 mm above rail table; gap to rail web: 45 ±5 mm.
- Cabling: 1.5 mm², 2-core twisted, shielded, 500 V grade; resistance loop ≤ 25 Ω.
- Earthing: TD body to structure earth ≤ 4 Ω; evaluator to power earth ≤ 1 Ω.
- Reset procedure:
a) PRE (Pre-reset) – given by Station Master/Controller.
b) SR (Section Reset) – given by Cabinman after physically inspecting the section.
c) Keys sealed in interlocked key-box; event logged in Data Logger.
10. Recent Updates (2022-24)
- Indigenous “NavAC” developed by BHEL & DMW under “Make-III” category; trials completed at Lucknow NR (2023).
- RFID-based redundant tag introduced by RDSO to auto-correct wheel-sensor drift – pilot at Tundla Jn. (2022).
- Integrated CountercumTC Module (ICTM) – hybrid card giving fail-safe OR of AC & TC outputs; approved May 2023.
- Digital twin: cloud-based Axle Counter Health Monitoring System (ACHMS) deployed on Eastern DFC; predicts MTBF with 97 % accuracy.
- Green initiative: solar-powered 24 V 40 Ah LiFePO4 battery pack adopted in NWR desert sections (Jaisalmer–Barmer) saving 1.2 lakh litre diesel/year.
11. Quick-Fire Data for MCQs
- First Indian AC section: Igatpuri Cabin-I siding (Central Railway) – 1984.
- Longest AC section: Son Nagar–Dildarnagar 23.4 km (ECR) – commissioned 2020.
- Total AC sections on IR (Mar 2024): 8 750 (siding 60 %, running 40 %).
- Target under Rashtriya Rail Sanraksha Kosh (RRSK): 15 000 km by 2030.
- Cost per running km (2023 price level): ₹ 6–8 lakh (vs ₹ 4 lakh for 50 Hz TC).
- Power consumption per section: 6 W (outdoor) + 12 W (indoor) = 18 W (1.5 W per TC section).
- World’s highest axle counter: 5 100 m amsl – Qinghai–Tibet railway (China) – CSEE system.
BETA
