When a longer stop travels down the line
See why an extended station stop can be absorbed in one situation and propagate in another. This is a conceptual sequence, not a forecast for a particular incident.
One stop takes longer
A busy station stop lasts longer than the plan allowed. More time is needed at the passenger interface before departure is ready. There is no universal number of seconds that causes a network problem: the consequence depends on where the stop occurs, the following service and the spare margin already in the plan.
Compare the actual time occupying the platform with the time allowed in the service plan.
A longer stop matters through its relationship to the next movement.
Read the complete journey
- 1
One stop takes longer
A busy station stop lasts longer than the plan allowed. More time is needed at the passenger interface before departure is ready. There is no universal number of seconds that causes a network problem: the consequence depends on where the stop occurs, the following service and the spare margin already in the plan.
Notice: Compare the actual time occupying the platform with the time allowed in the service plan.
Explore Platforms and dwell time
Why: A longer stop matters through its relationship to the next movement. - 2
Use the available margin
The timetable may contain enough room to absorb some of the variation. If the train clears the constraint before the following service needs it, that following train may be unaffected. The scenario on the system overview isolates this threshold by comparing a repeated station constraint with a chosen planned headway.
Notice: Identify the unused time before the following train needs the shared constraint.
Explore Timetables and headway
Why: That margin determines whether the first variation reaches the next train. - 3
Keep the next movement protected
If the space ahead is not available, the following train’s movement remains restricted. That train cannot solve the delay by ignoring its permitted separation. The same protection that keeps movements compatible also makes the operational consequence visible: the next train waits or approaches under the applicable limits, while its own arrival time can change.
Notice: Notice that the following train’s planned departure does not make occupied space available.
Explore Signals and separation
Why: Protection requirements continue to apply during a delay. - 4
Meet other shared routes
Further along, a delayed train may reach a junction at a different time from the plan. Another route can already need that space. This does not guarantee a cascade, but it creates another opportunity for one movement to affect another. The network’s layout determines which conflicts are possible and which alternative paths exist.
Notice: Trace where two routes need the same junction rather than simply comparing their station arrival times.
Explore Points and interlocking
Why: A delay can change the order in which shared route capacity is used. - 5
Recover a coherent service
Operational teams assess the disturbance and coordinate recovery. Restoring regular spacing may require service changes, depending on the line and incident. Passenger information needs to follow those decisions. A delayed train may also be the vehicle for a later departure, so the effects can continue after the initial station has cleared.
Notice: Look at the delayed train’s next duty as well as its current destination.
Explore Operations and recovery
Why: A disturbance can travel with a vehicle or crew after the original location has cleared.
Why can the same extra station time leave one following train unaffected but delay another?
Reveal the explanation
The effect depends on available margin and shared constraints. If the first train clears before the next needs the route, the variation may be absorbed. If the constraint remains occupied, the next movement must wait; later resource and junction conflicts can extend the effect.
Sources & further reading
Original explanations informed by these sources. Diagrams are conceptual; local systems differ.
- Transit Capacity and Quality of Service Manual, Third Edition, Chapter 8: Rail Transit Capacity Transportation Research Board / National Academies · 2013 · Source & rights record
- The timetable Network Rail · Not stated · Source & rights record
- Signals explained Network Rail · 2018-09-05 · Source & rights record
- Signals and points failure Network Rail · Not stated · Source & rights record
- Knock-on delays Network Rail · Not stated · Source & rights record
- How we keep you updated when there’s a delay Network Rail · Not stated · Source & rights record
Another way through the system.
Take a train across the city
Follow the passenger, then reveal the systems underneath.
Follow a train’s energy
From the electricity supply to acceleration, then braking.
Before the first passenger boards
Follow a train from preparation to its first useful departure.
After heavy rain, before normal service
Follow water through the railway’s connected dependencies.