One morning, four systems
An original teaching journey through a fictional city with a treated-water network, electrically powered homes, internet access and an electric urban railway. All services are available this morning. Their connections are chosen to explain dependencies, not to describe every city or trace the same physical resources from end to end.
Begin with water already prepared
Before leaving, our traveller fills a bottle from the tap. This fictional supply has already collected, treated and stored the water. Pressure moves it through the distribution network to the building. Electricity may have been used earlier for treatment and pumping, so the water arriving now does not require every upstream pump to run at that exact moment. Stored water links different times of day.
Trace the tap backwards to both a treated-water store and the equipment that replenishes it.
A service available now can depend on work and energy supplied earlier.
Read the complete journey
- 1Water
Begin with water already prepared
Before leaving, our traveller fills a bottle from the tap. This fictional supply has already collected, treated and stored the water. Pressure moves it through the distribution network to the building. Electricity may have been used earlier for treatment and pumping, so the water arriving now does not require every upstream pump to run at that exact moment. Stored water links different times of day.
Notice: Trace the tap backwards to both a treated-water store and the equipment that replenishes it.
Explore Distribution & the tap
Why: A service available now can depend on work and energy supplied earlier. - 2Electricity
Add breakfast to the electrical load
The kettle adds an electrical load while it heats water. Elsewhere, pumps, network equipment and trains are also using electricity, although they need not share the home's local supply circuit. The kettle's power describes its rate of energy use; the time it operates determines the energy transferred. The city combines many such changing loads, rather than assigning one generator to this breakfast.
Notice: Compare the brief kettle load with supporting equipment that may operate for much longer.
Explore The appliance & its circuit
Why: Shared electrical supply connects activities whose timing and energy requirements differ. - 3Water
Leave a second water journey behind
Water used at the sink enters the wastewater system while the traveller moves towards the station. In our chosen layout, gravity carries sewage downhill before a lift station raises it towards treatment. That pump creates another electrical dependency. Drinking-water delivery and wastewater removal are distinct services: a working tap does not establish that every downstream sewer or treatment process is operating normally.
Notice: Follow the drain towards a low point and a separate pumped section, rather than back into the drinking-water pipe.
Explore Sewers & lift stations
Why: A useful water service includes removal and treatment as well as delivery. - 4Internet
Ask the network about the journey
The traveller checks a service page using home Wi-Fi. The phone, local network equipment and the provider's wider connection all contribute to reaching that page. A charged phone supplies only its own energy; it does not power the home router or distant equipment. This morning those supporting devices are available, allowing the request to leave the home and reach the service hosting the information.
Notice: Separate the phone's battery indicator from the powered equipment along its connection.
Explore The access network
Why: An online action depends on an end-to-end path beyond the device in your hand. - 5Internet
Receive information about a different system
The service returns a page describing the railway. Its usefulness depends on the information supplied and how recently it was updated, as well as successful delivery over the internet. A page that loads correctly can still describe an earlier service state. Passenger information and railway movement permissions serve different purposes; this fictional public website does not control whether a train may enter a section of track.
Notice: Look for the difference between a page arriving successfully and its service information being current.
Explore Web servers & HTTP
Why: Transport information is a connection between systems, not proof of railway readiness. - 6Urban rail
Turn a railway service into a usable trip
At the station, the traveller follows the entrance, circulation route and platform access. Our example includes an electrically powered lift on the required route. An operating train alone would not make this particular journey usable if that lift were unavailable and no suitable alternative existed. Accessibility joins the train service to the passenger's actual starting point and destination, including the final route back to the street.
Notice: Trace a continuous passenger route through the station, including the lift assumed in this example.
Explore Station access
Why: Service availability must be assessed from the user's complete journey. - 7Urban rail
Convert electrical supply into movement
The train accelerates using its railway traction supply. This is another connection to the electricity system, through equipment designed for the railway's voltage and collection arrangement. A lit station and an energised traction supply are separate observations, because different electrical circuits can serve them. In this morning's scenario, traction equipment, track and movement permissions are all available, so the train can continue towards its destination.
Notice: Follow the traction supply to the train separately from the circuits serving station lighting.
Explore Traction power
Why: Different functions within one station can depend on different electrical arrangements. - 8Urban rail
See the commute as several connected services
The traveller arrives while the railway continues coordinating trains and passenger information. Water storage, electrical supply, communications and transport have supported different parts of one ordinary morning. None followed exactly the same route or timetable. The completed trip depends on their interfaces working when needed: water at home, a reachable information service, usable station access and a train able to move within its operating constraints.
Notice: Name one interface between systems at each stage of the completed trip.
Explore Operations and recovery
Why: Following a person's activity reveals dependencies that separate network maps can hide.
Why can a charged phone and a running train still be insufficient for this traveller's complete journey?
Reveal the explanation
The phone still needs available network equipment and a reachable, useful information service. The traveller also needs a usable route through both stations; in this example that includes a lift. Device power and train movement each establish only one part of the complete service.
Sources & further reading
Original explanations informed by these sources. Diagrams are conceptual; local systems differ.
- Infrastructure Resilience Planning Framework (IRPF) Cybersecurity and Infrastructure Security Agency (CISA) · 2025-03 · Source & rights record
- Water System Planning Guideline Sydney Water · 2014-09 · Source & rights record
- Protecting Water Quality Through Distribution System Pressure Management US Environmental Protection Agency · 2021-12 · Source & rights record
- Where your sewage goes Melbourne Water · 2023-05-09 · Source & rights record
- Measuring electricity U.S. Energy Information Administration · Not stated · Source & rights record
- How electricity is delivered to consumers U.S. Energy Information Administration · Not stated · Source & rights record
- Stay connected in severe weather Ofcom · 2026-10-22 · Source & rights record
- RFC 1122: Requirements for Internet Hosts — Communication Layers IETF / RFC Editor; R. Braden (editor) · 1989-10 · Source & rights record
- RFC 9110: HTTP Semantics IETF / RFC Editor; R. Fielding, M. Nottingham, and J. Reschke (editors) · 2022-06 · Source & rights record
- Plan an accessible journey Transport for London · Not stated · Source & rights record
- Using TfL Go for step-free journeys Transport for London · Not stated · Source & rights record
- Electrification Network Rail · Not stated · Source & rights record
- Signals explained Network Rail · 2018-09-05 · 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.
One interruption, different clocks
Discover why dependent services do not all stop together.
When rain crosses the network map
Follow one shared hazard across boundaries drawn for different services.
Power is back. What else must recover?
Follow service restoration through several independent conditions.