Signal Types – Railway General Knowledge Capsule

1. Definition & Purpose

Railway signals are visual communication devices that convey mandatory instructions to drivers regarding:

• Movement authority
• Speed limits
• Route settings
• Emergency situations

Signals replace verbal orders, ensure headway, and are the primary safety layer in the “fail-safe” signalling system.

2. Classification Tree (Indian Railways – IRS: S-23 & S-36)

Signals
├── Fixed Signals
│   ├── Stop Signals
│   │   ├── Outer (OS)  
│   │   ├── Home (HS)  
│   │   ├── Starter (SS)  
│   │   └── Advanced Starter (AS)
│   ├── Permissive Signals
│   │   ├── Caution (CS)  
│   │   ├── Warner (WS)  
│   │   └── Calling-on (CO)
│   ├── Repeater Signals (RP)
│   ├── Routing Signals
│   ├── Shunt Signals
│   │   ├── Dwarf (Mini) – 2-aspect
│   │   ├── Position-light – 3-lamp
│   │   └── LED cluster – 2-aspect
│   └── Miscellaneous
│       ├── Gate Signals
│       ├── Junction Signals
│       └── Slip-Siding Signals
├── Semi-Fixed Signals
│   ├── Detonating Plates (21 m spacing)
│   └── Fog Signals (Hand / Explosive)
└── Temporary / Hand Signals
    ├── Red & Green Flags
    ├── Hand Lamps
    └── Radio-communication (Walkie-Talkie)

3. Technical Specifications (2024)

4. Historical Milestones

5. Aspect Sequence & Indications (MACLS – 4 aspects)

Additional flashing aspects (Automatic Block):

• Flashing Yellow – Approach block section at 30 km/h
• Flashing Double Yellow – Approach flashing yellow ahead

6. Current Status & Recent Updates (2024)

• Total signals on IR: 66 400 (as on 01.04.2024)
• LED converted: 56 800 (≈ 85.5 %)
• Mechanical left: 9 600 (mostly branch lines & non-interlocked cabins)
• New procurement: 3 200 LED colour-light sets under “Mission 3000” (2024-26)
• Standard: IRS:S-36/2023 superseded S-23/2004; mandates IP-66 housing, <5 W per aspect, <1 cd stray light.
• RFID tag: Every signal mast to have passive UHF tag for asset tracking (implemented since Jan 2023).

7. Quick-Fire Facts (Exams)

1. Smallest signal: Dwarf shunt – 0.6 m height.
2. Tallest signal: 15 m cantilever at Bharuch Jn (WR) for 6-line visibility.
3. Longest stop signal spacing: 1.4 km on Ghat sections (Konkan Railway).
4. Detonating plate: 21 m apart, 10 explosions/minute at 60 km/h.
5. Calling-on signal: white light below main signal, permits 15 km/h into occupied block.
6. Repeater signal: “R” board & offset spectacle to avoid confusion.
7. Junction routing signal: three arms (semaphore) or 3-way route indicator (colour-light).
8. Slip siding signal: “SS” board & miniature red to protect against rolling-back.

8. 15+ MCQs with Answers

Revise → Recall → Reproduce