Interlocking – The Heart of Safe Train Operations

Interlocking is the safety logic system that prevents a train from getting a “proceed” signal unless the route is set, locked, proved and the conflicting moves are isolated. It is the brain of the signalling system and every exam—NTPC, JE, ALP, Group-D—carries 3-5 questions on it every year.

1. Technical Fundamentals

Essential Conditions before a signal is taken “OFF” (permanent way & signalling)

1. All points in the route are correctly set & locked.
2. All facing points are locked individually (clip lock/detector lock).
3. The entire route including overlap (180 m for BG, 120 m for MG) is clear of trains.
4. No conflicting route is set or locked.
5. Level-crossing gates (if within the route) are closed & locked in road-traffic direction.
6. The signal cannot display an aspect less restrictive than the track ahead permits.

Types of Interlocking

2. Facts, Figures & Specifications (must-memorise)

• Overlap length: 180 m BG / 120 m MG / 150 m NG (SEM-2019, para 7.39)
• Minimum flank-protection distance: 50 m for stations where speed > 130 km/h
• Point machine stroke: 143 mm (BG) / 120 mm (MG) — IRS:S-19
• Detection contact gap: ≤ 0.5 mm for lock detection
• Crank-handle interlock: Provided to prevent manual operation of points when route is locked
• Signal overlap timing: 4 s for LED signals, 7 s for filament lamp (to prove healthy)
• Relay contact rating: 700-type—silver cadmium oxide, 2 A @ 24 V DC
• SSI mean time between failures (MTBF): ≥ 1.1 × 10⁵ h (RDSO spec IRS:S-99)
• First all-colour-light station in India: Churchgate (WR) – 1928
• First RRI in India: Byculla (CR) – 1928 (electro-mechanical); first all-relay RRI – Bythahalli – 1959
• Longest lever frame in India: 180 levers, Howrah (ER) – 1926 (still preserved)
• Total stations with interlocking (2023): 7 215 out of 8 124 stations (89 %)
• Stations upgraded to CBI/SSI (2023): 1 847
• Stations still on pure mechanical (2023): 312 (mostly in NFR & SECR)

3. Historical Milestones

4. Current Status & Recent Updates (2022-24)

• RDSO has frozen new RRI supply vide letter no. RB/SE/Sig/Policy/22 dated 14-01-2022; only SSI/CBI to be provided.
• “One-Station-One-Product”—Indian Railways has started procuring 2-out-of-3 CBI from three approved vendors:
  • Siemens India (S7-400F-H)
  • Alstom India (SmartLock)
  • Bharat Electronics (BEL-Rakshak CBI)
• Gati-Shakti corridors: 100 % CBI with IP-based track-side fiber; silicon-rubber axial detection rods introduced to cut maintenance.
• Digital Interlocking Log (DIL)—New feature in CBI to store last 30 days event log with 1 ms resolution; helpful for accident inquiries.
• Provision of “Auto-Route Setting (ARS)” in CBI—reduces setting time from 8 s to 2 s; commissioned at New Delhi, Mumbai Central, Patna, Secunderabad.
• Kavach (ETCS-L1) overlap: When Kavach is enabled, overlap requirement is reduced to 120 m because train continuously supervises speed.
• Green Initiatives: LED signals only since 2018—saves 32 MU per annum.
• Railway Board circular (2023) mandates CCTV integration with CBI so that cabin operator can view point detection in real-time.

5. 15+ Memory Code MCQs

Q1. The minimum length of overlap for BG station where speed ≤ 110 km/h is
A. 120 m B. 180 m C. 240 m D. 360 m

Q2. First station in India provided with Solid-State Interlocking (SSI) was
A. New Delhi B. Borivili C. Tundla D. Byculla

Q3. The maximum stroke of a standard BG point machine is
A. 120 mm B. 143 mm C. 165 mm D. 220 mm

Q4. Which relay type is universally used in Route-Relay Interlocking?
A. Q-series B. 700-type C. 930-type D. 1000-type

Q5. In CBI, the voting logic normally adopted is
A. 2-out-of-2 B. 2-out-of-3 C. 1-out-of-2 D. 3-out-of-3

Q6. The crank-handle interlocking is provided to ensure
A. Signal cannot be taken OFF unless crank handle is inside
B. Crank handle cannot be removed unless route is locked
C. Point cannot be moved unless crank handle is removed
D. Crank handle is sealed

Q7. Overlap is reckoned from
A. Tip of switch rail B. heel of switch rail C. fouling mark D. signal post

Q8. Which of the following is NOT a standard aspect of a 4-aspect colour-light signal?
A. Green B. Double Yellow C. Yellow D. Red

Q9. The minimum flank protection distance for stations with speed > 130 km/h is
A. 30 m B. 50 m C. 120 m D. 180 m

Q10. Which of the following corridors is being commissioned with ETCS Level-2?
A. Kolkata Metro B. Golden Quadrilateral C. Kalka-Shimla D. Nilgiri Mountain

Q11. The event log storage duration in Digital Interlocking Log (DIL) of CBI is
A. 7 days B. 15 days C. 30 days D. 90 days

Q12. A mechanical lever frame with 120 levers will normally be housed in
A. Single-tier cabin B. Double-tier cabin C. Ground frame D. Shunting frame

Q13. Which of the following is the latest interlocking technology approved by RDSO in 2022?
A. RRI B. Panel C. CBI D. Token block

Q14. When Kavach is provided, the overlap requirement is reduced to
A. 50 m B. 120 m C. 180 m D. 240 m

Q15. The first colour-light signal on Indian Railways was introduced at
A. Howrah B. Madras C. Churchgate D. Delhi

Q16. Which component proves that points are locked in an RRI system?
A. Track relay B. Lock detection contact C. WCR D. HR

Q17. The mean time between failures (MTBF) specified for SSI is
A. ≥ 1.1 × 10⁴ h B. ≥ 1.1 × 10⁵ h C. ≥ 1.1 × 10⁶ h D. ≥ 1.1 × 10⁷ h

Pro-Tip: Draw a timeline chart and a comparison table in your notes; 60 % of interlocking questions are fact-based and direct.