Railway Signalling Systems
Railway Signalling Systems
Overview
Indian Railways uses an exclusive colour-light signalling system (no semaphore remaining on running lines) governed by the “Rules for Opening of Railways” 2021 and the Signal & Telecommunication (S&T) Manual. The aim is to maintain minimum head-way, ensure absolute block, permit higher sectional speeds & enable automatic train protection (ATP) through indigenous and world-class technologies.
Key Facts & Figures
| Fact | Detail |
|---|---|
| 1. First colour-light signal | 1928 – GIPR (Borivali-Virar) |
| 2. Present signalling standard | 3-aspect / 4-aspect Multiple-Aspect Colour-Light (MACL) |
| 3. Cab signalling technology | ETCS Level-2 (LDQ & WDFC) & Kavach (indigenous ATP) |
| 4. Highest speed under signalling | 160 km/h (Delhi-Mumbai & Delhi-Howrah RRTS) |
| 5. Automatic signalling sections | >5,400 Rkm (as on 31.03.2024) |
| 6. Route relay interlockings | >1,100 (largest – Lucknow 1,280 routes) |
| 7. Panel interlockings | >4,900 (being converted to RRI/ECI) |
| 8. Electronic interlocking (ECI) | >1,050 supplied by RSEB/IRSI/DMRC consortium |
| 9. Block proving by axle counters (BPAC) | >6,700 block sections |
| 10. Automatic train protection cover | 3,038 Rkm (Kavach) target 100 % A & B routes by 2030 |
| 11. Train protection warning system (TPWS) | 1,100 Rkm (Calcutta-Chennai & Mumbai-Kalyan) |
| 12. Standard signal spacing | 1 km for 3-aspect; 2 km for 4-aspect (max) |
| 13. Signal voltage | 110 V DC for filament LEDs; 24 V for LED clusters |
| 14. Signal sighting distance | 400 m min (plain track); 200 m in curved territory |
| 15. Longest automatic section | 225 km – Virar-Dahanu (WR) |
| 16. First station with LED signals | Mumbai Central (2012) |
| 17. Block section occupancy time | 5 min (MACL) vs 11 min (two-aspect) |
| 18. Fail-safe principle | All relays drop to danger when de-energised |
| 19. Aspect sequence violation | Recorded in data logger; 48 h retention |
| 20. Interlocking software audit | Independent Safety Assessor (ISA) certifies to CENELEC SIL-4 |
Important Points
- Absolute block system – no second train enters until first has cleared block + 150 m overlap.
- Stop-signal: red (danger), yellow (caution), green (proceed); distant-signal: yellow (caution), green (proceed).
- Calling-on signal – miniature yellow, permits 15 km/h after stopping at danger.
- Repeater signal – offset mounting, letter “R” plate, repeats aspect of main signal.
- Shunt signal – two white horizontal bars (proceed) & two red vertical bars (stop).
- Route indicators – junction type (rows of white lamps) & stencil type (alphanumeric).
- Automatic block signalling (ABS) needs continuous track-circuiting or axle-counters + track vacancy detection.
- Centralised Traffic Control (CTC) on 3,800 Rkm allows remote signalling from divisional control office.
- Data logger – records 64 parameters; mandatory for RRI/ECI stations.
- Signal overlap – 120 m for 100 km/h; 180 m for 130 km/h; 360 m for 160 km/h.
- Engineering allowance time – 2 min for single-line & 1 min for double-line during block working.
- Kavach works on 825 MHz TDD radio; supports 4 km head-way @ 160 km/h.
- LED signal unit draws <20 W; life 1,00,000 h; visibility 1 km in clear weather.
- Signal passed at danger (SPAD) – any part of train > 2 m beyond stop signal without authority.
- Railway Board code for signalling – “IRSEM” (Indian Railway Signal Engineering Manual) 2021 edition.
Frequently Asked in Exams
- Difference between 3-aspect and 4-aspect signalling & impact on line capacity.
- Functions of axle-counter versus track-circuit in block proving.
- Working principle of Route Relay Interlocking vs Electronic Interlocking.
- Kavach specifications: frequency, speed support, head-way achieved.
- Automatic signalling conditions as per GR 3.48 & special instructions.
Practice MCQs
Question:01 Which signalling aspect allows the driver to enter the block at 160 km/h?
A) Yellow with “160” speed indicator board (3-aspect)
B) Double Yellow with “160” speed indicator board (4-aspect)
C) Green with “160” speed indicator board (4-aspect)
D) Flashing Green with “160” speed indicator board (4-aspect)
Show Answer
Correct Answer: C
Explanation: In 4-aspect signalling, a Green aspect with a “160” speed indicator board authorises the driver to enter the block at the maximum permitted speed of 160 km/h.
Question:02 Minimum overlap length provided beyond a stop signal for 130 km/h is –
A) 120 m
B) 150 m
C) 180 m
D) 200 m
Show Answer
Correct Answer: C
Explanation: As per Indian Railway standards, the minimum overlap length beyond a stop signal for a maximum speed of 130 km/h is 180 m.
Question:03 Kavach ATP uses which radio band?
A) 700 MHz FDD
B) 825 MHz TDD
C) 900 MHz FDD
D) 1.8 GHz TDD
Show Answer
Correct Answer: B
Explanation: Kavach ATP operates on the 825 MHz TDD (Time Division Duplex) radio band for its train protection communications.
Question:04 First colour-light signal on Indian Railways was commissioned in –
A) 1925 (GIPR)
B) 1928 (GIPR)
C) 1930 (GIPR)
D) 1932 (GIPR)
Show Answer
Correct Answer: B
Explanation: The first colour-light signal on Indian Railways was brought into service in 1928 on the Great Indian Peninsula Railway (GIPR).
Question:05 The letter plate mounted on a repeater signal displays which alphabet?
A) P
B) R
C) S
D) L
Show Answer
Correct Answer: B
Explanation: As per Indian Railway signalling code, a repeater signal is identified by the letter “R” painted on its plate.
Question:06 Which of the following is NOT a stop signal?
A) Home signal
B) Starter signal
C) Advanced starter signal
D) Distant signal
Show Answer
Correct Answer: D
Explanation: A Distant signal only gives advance warning and does not require the driver to stop at it; hence it is NOT a stop signal.
Question:07 Maximum permissible spacing between two consecutive stop signals under 4-aspect Multiple Aspect Colour Light (MACL) signalling is
A) 1 km
B) 1.5 km
C) 2 km
D) 2.5 km
Show Answer
Correct Answer: C
Explanation: As per Indian Railway Signalling Rules, the maximum distance allowed between two stop signals in 4-aspect MACL territory is 2 kilometres to ensure adequate braking distance and signal visibility.
Question:08 BPAC stands for –
A) Block Proving by Axle Counters
B) Brake Power Assessment Control
C) Bogie Performance Analysis Circuit
D) Buffer Pressure Adjustment Component
Show Answer
Correct Answer: A
Explanation: BPAC is the abbreviation for Block Proving by Axle Counters, a system used in railways to verify track section occupancy through axle-counting technology.
Question:09 Which Indian interlocking installation handles the greatest number of routes?
A) Mumbai CST
B) Lucknow
C) Howrah
D) New Delhi
Show Answer
Correct Answer: B
Explanation: Lucknow interlocking currently controls 1,280 routes, the highest count in India.
Question:10 If a train passes a danger signal by more than 2 m without authority, it is termed –
A) Over-run
B) SPAD
C) Signal breach
D) Red-zone violation
Show Answer
Correct Answer: B
Explanation: When a train goes past a danger (red) signal by more than 2 metres without authority, the incident is classified as SPAD—Signal Passed At Danger.
BETA
