Follow the Links: How Engineers Keep Track of Safety

A promise needs a trail

Imagine a passenger train approaching a red signal. Its automatic braking system must stop it before it reaches a dangerous point.

Engineers turn that goal into requirements: precise statements of what the train must do. One might specify the maximum stopping distance under defined conditions, such as speed, load and wet rails.

But writing a requirement is only the start. How do you know the design addresses it? Where is the test result? What happens when something changes?

Requirements traceability means keeping clear links between a requirement, its purpose, the design that addresses it and the evidence used to check it.

Connect the whole journey

A train’s lifecycle is its whole journey, from the first idea through design, building, operation and eventual retirement. Traceability needs to follow that journey, not stop when the train leaves the factory.

For the braking requirement, engineers might connect:

• The safety goal: avoid passing a danger point.
• The requirement: stop within a stated distance under stated conditions.
• The design: brakes, sensors and control software.
• The evidence: calculations, inspections and test results.
• The maintenance instructions: checks that help keep the brakes working correctly.

Engineers can follow these links in both directions. Starting with a requirement reveals its supporting evidence. Starting with a test reveals which requirement it checks.

When one change spreads

Suppose heavier batteries are fitted to the train. More mass could affect stopping distance.

The links help engineers find the braking requirements, calculations and tests that may need review. They can then decide what must change and what needs testing again.

This is why a tick beside “tested” is not enough. The result must identify which version of the train was tested and under what conditions. Evidence from an earlier design may no longer apply.

Try the missing-link check

Pick one requirement in a project, even a school-built robot. Ask: why does it exist, what addresses it, and what evidence checks it?

If an answer is missing, you have found a gap worth investigating. A link alone does not prove safety, but it helps people find and judge the evidence.

Engineer-approved AI can help suggest links and flag gaps, while engineers check the results.