The subway network can be split into 4 main sub systems, those being design, manufacturing and maintenance, operation and customers. The design process initially involves designing the desired rail network, however once the entire system in in operation the design sub system involves the constant re design of features and parts caused by arising issues as well as updating the system to use newer technologies. This subsystem requires many disciplines of engineering including electrical which designs many of the electrical features including the main power system responsible in powering of the overall system , civil which designs structures such as stations, bridges and tunnels underneath the city  and mechanical which is involved in the design of railway vehicles .
The manufacturing and maintenance subsystem involves the building of everything required for the system to function from manufacturing the trains themselves to constructing the network of rails, tunnels and stations they travel through. This is all continuously maintained to assure it is reliable and fully functioning to allow for the system to be operated. One important engineering discipline involved in the manufacturing subsystem is manufacturing engineering which with the aid of many other engineering disciplines is able to turn raw materials into new products such as train carts, railway tracks etc . The operation process is the subsystem that the customers or users of this system rely heavily upon. The operation of this system involves using all the parts of this system in order to fulfil its function of transporting its users. The operation process involves constant monitoring of the system in order to allow for a schedule as well as maintain safety and assure the system is functioning correctly. This all requires many operators such as traffic controllers, drivers and conductors. Software Engineering is a very important part of the operation process as it is responsible for developing software to track rail carts, display arrival times and operate automated train supervision .
The final subsystem of this system is the customers or the users. This system includes all the users of the rail system and as the main purpose this system is to provide customers with transportation this subsystem like every other relies heavily upon all the other subsystems in order to allow the rail system to function.
The main subsystems or the systems key elements interact strongly with each other and rely heavily upon each other in order for the system to function. The whole purpose of the rail transportation system is to create a more efficient, convenient and more simple way for people to travel to and from a desired location. A high level of intelligent communication and interactions between the subsystems allows the system to meet its desired outcomes. As transportation is the users need there is a strong interaction between the customers and the operation subsystems. In order to meet customers’ needs the operation of this system needs to be working efficiently, this requires all the parts involved in this subsystem to be running proficiently as any dilemmas within this subsystem can impact the customers causing problems such as delays and accidents which can lead to many complications with the system as a whole .
In order to meet customers needs there is a strong interaction between the operation and the manufacturing and maintainance subsystems. In order to operate the system there initially needs to be parts that have been manufactured such as tracks, stations and trains. However once the system is running constant maintenance on parts as well as the manufacturing of new parts are needed in order to allow to allow for the operation of the system to run efficiently . Problems in the manufacturing and mainanace system can furthur lead to operation issues that can have a devastating impact on the customers. To assure that apropriate parts are made the manufacturing and maintainance subsystem relies on the design subsystem. This subsystem is extremely important in assuring that features of the system are designed apropriatly eliminating any potential risks during the manufacturing and operating stages .
Finally the customer subsystem has strong interaction between all the other main susbystems. Actions and the behaviours of the customers infuluences and changes the ways all the subsystems interact. More users require more operation which leads to the need of more manufacturing, maintainence and design. Less users can potentially reverse this effect. In order to create a sustainable system the key elements need to rely heavily upon each other .