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There are many systems such as computer systems, the ecosystem, and traffic and transport systems to mention only three of them. I would even claim that everything is part of a system as well as comprises subsystems itself. A motor for example is a subsystem of a car what is a subsystem of road traffic. This is a subsystem of the ecosystem. If one continues like that, one shifts from science to philosophy. I am not going to do that here.
A system is a set of various elements collaborating as a unit to perpetuate a defined behaviour. The chronological behaviour of a system is called a process.
The railway describes such a system consisting of the elements vehicle, infrastructure, and as a connection between those two the operation of railway.
The system is:
Open to its environment, that is, there are interactions between system and environment.
Dynamical because the system variables are varying chronologically
Stochastically because even if identical conditions are provided, are subsequent states of the system only describable by probability conclusions
Instable because the system is already tilting at very little disturbances.
Necessarily there are variables within the system behaving different from the above mentioned. The mass for example of a power unit is nearly constant. But the total mass of the train is subject to frequent changes already and thus has to be regarded as dynamical. The same applies for other properties.
The system railway consists of the subsystems rail vehicle, rail infrastructure and as a link railway operation. The subsystems have to work together and thus the inherent interfaces have to be designed well. Some of the interfaces are: