Signal Spacing Calculator

Line Capacity Starts With How Close Trains Can Safely Follow Each Other

Signal spacing determines the minimum safe distance between successive trains on the same line — and by extension, the maximum number of trains that can run per hour. Get it wrong in one direction and trains run too close together to guarantee a full stop before a conflict; get it wrong in the other and the line carries far less traffic than it could. This calculator ties braking distance directly to headway and hourly capacity.

The Formula

Braking distance uses the same physics as a standalone stopping-distance calculation; signal spacing then adds the length of the train itself (so the rear clears the signal before the next one is approached) and any additional safety margin.

Where This Is Useful

• Line capacity planning — the headway this formula produces directly caps how many trains per hour a signaled section can carry.
• Signal placement design — confirms that fixed signals are spaced far enough apart for the fastest train expected to use the line to stop safely.
• Speed and frequency trade-offs — shows how raising line speed increases braking distance and can reduce the trains-per-hour capacity unless spacing is also adjusted.

Worked Example

A train at 120 km/h, decelerating at 0.8 m/s², 200 m long, with a 50 m safety margin:

This capacity figure is a theoretical maximum based purely on braking physics; real timetables build in additional buffer for signal reaction times, dwell at stations, and operational variability.

How to Use This Calculator

1. Enter the Train Speed in km/h.
2. Enter the Deceleration Rate in m/s².
3. Optionally enter the Train Length in meters.
4. Optionally enter a Safety Margin in meters.
5. Select Calculate to get minimum signal spacing, headway, and theoretical line capacity in trains per hour.

Related Calculations

This calculator builds directly on the Braking Distance Calculator. For the speed feeding into both, see the Train Speed Calculator.