Overview
Line Capacity determines how many trains per hour can safely operate on a railway line.
Headway is the minimum time interval between two consecutive trains for safe operation.
Both are fundamental concepts for:
- Metro and commuter systems
- High-speed lines
- Freight corridors
- Mixed traffic railways
Understanding them is essential for designing efficient, safe train operations.
What Is Headway?
Headway = Time gap between two trains on the same track, in the same direction.
For example:
If the headway is 3 minutes, the maximum theoretical frequency is 20 trains/hour.
Headway is influenced by:
- Signalling system
- Train braking distance
- Line speed
- Dwell time at stations
- Train type (freight vs passenger)
- Driver behavior (in non-automatic systems)
Types of Headways
1. Fixed Block Headway
The line is divided into “blocks.”
Only one train can be inside a block at any time.
Used in:
- Conventional mainlines
- Freight routes
- Many suburban lines
Limitation: Larger blocks = larger headway.
2. Moving Block (CBTC / ETCS L3 style)
Headway depends on:
- Real-time train position
- Its actual braking distance
Used in:
- Modern metro systems
- Some driverless operations
Enables very low headways (e.g., 90–120 seconds).
3. Timetable Headway
Even if signalling allows shorter intervals, timetable spacing may require more time for:
- Overtakes
- Platform sharing
- Junction conflicts
What Is Line Capacity?
Line capacity = Maximum number of trains the line can handle per hour, per direction, safely and reliably.
Capacity depends on:
- Headway
- Signalling technology
- Speed and acceleration of trains
- Junction layouts
- Turnout speeds
- Dwell times
- Mix of train types (freight vs high-speed)
- Operational rules (country-specific)
Factors Affecting Headway & Capacity
1. Signalling System
Better signalling = lower headway = higher capacity.
| Signalling Type | Typical Headway | Capacity (tph) |
| Lineside signals (basic) | 5–8 min | 7–12 tph |
| 3-aspect/4-aspect | 3–5 min | 12–20 tph |
| ETCS L2 / ATP | 2–3 min | 20–30 tph |
| CBTC (moving block) | 90–120 sec | 30–40+ tph |
| UTO (driverless metro) | 75–100 sec | 36–48+ tph |
tph = trains per hour
2. Train Braking Distance
Fast trains with long braking distances require larger headways.
Slower metro trains can run closer together.
Example:
- High-speed line → headway may be 5–7 minutes
- Metro line → can run trains every 90 seconds
3. Dwell Time at Stations
Longer station stops reduce capacity.
Examples:
- Metro lines: 20–45 seconds
- Commuter trains: 30–60 seconds
- Long-distance: 1–5 minutes
Crowded platforms → more dwell time → lower capacity.
4. Turnout Speeds & Junction Layouts
Junction conflicts are major bottlenecks.
- Faster turnout speed (e.g., 80 km/h vs 30 km/h) → higher capacity
- Grade-separated junctions → no conflicting movements
- Flat junctions → trains must cross each other’s paths → lower capacity
5. Mixed Traffic (Freight + Passenger)
If a line has:
- Fast trains
- Slow freight
- Frequent stoppers
→ Headway increases significantly due to speed differentials.
Dedicated freight or passenger corridors improve capacity greatly.
6. Automation Level
Automation reduces variability.
Levels:
- ATO (Automatic Train Operation)
- ATP (Automatic Train Protection)
- UTO (Unattended Train Operation)
More automation → shorter headways → predictable operation.
Practical Examples
Metro Systems
- Typically the highest capacity railways
- CBTC + high acceleration
- Headways as low as 75–120 seconds
- Some lines reach 40+ tph
Examples include metros in Singapore, Hong Kong, Dubai, London, and Paris.
High-Speed Rail
- Long braking distances → naturally higher headways
- Usually 8–15 trains per hour per direction
- ETCS Level 2 or Chinese CTCS for safety
- Junctions avoided to prevent conflicts
Freight Corridors
- Heavy trains → long braking distance → larger headways
- Typically 4–8 trains per hour
- Prioritizes reliability over frequency
Examples: North American freight networks, European freight corridors, Indian DFC.
Mixed Traffic Mainlines
The hardest environment for capacity planning.
High-speed + commuter + freight together = severe constraints.
Solutions:
- Passing loops
- Overtake sections
- Dedicated tracks for suburbs
- Better signalling (ETCS/CTC)
How Capacity Is Improved
Railways improve line capacity by:
1. Upgrading signalling
Lineside signals → Automatic Block → ATP → CBTC (for metro)
2. Improving junctions
Grade separation, flyovers, high-speed turnouts.
3. Reducing dwell times
Better door systems, platform staff, improved passenger flow.
4. Increasing traction performance
Faster acceleration → trains clear stations sooner.
5. Using homogeneous traffic
Separate fast and slow trains.
6. Introducing automatic train control systems
Headway Formula (Simplified)
A general approximate relationship:
Capacity (tph) ≈ 3600 / Headway(seconds)
Example:
- 120-second headway → 30 trains/hour
- 90-second headway → 40 trains/hour
Common Questions
Why can metros run more trains than mainline railways?
Short braking distances + high acceleration + advanced signalling.
Why do freight lines have low capacity?
Heavy trains → long braking distances → long headways.
Why do high-speed trains need large separation even with advanced signalling?
Physics. Braking from 300 km/h requires 3–4 km.
What limits suburban railway capacity?
Long dwell times due to crowding.
Can increasing speed improve capacity?
Sometimes no — higher speeds can increase braking distance, reducing capacity.