What defines a double track system?

A double track system is defined by the presence of two tracks that allow trains to travel in opposite directions simultaneously. This configuration improves efficiency and safety, as it minimizes the likelihood of train collisions by providing dedicated paths for incoming and outgoing traffic. Having two tracks enables higher capacity and frequency of service on a railway line since trains can operate without having to wait for the track to clear before proceeding.

In contrast, the first option describes a setup where two tracks are aligned for the same direction, which does not meet the definition of a double track system in terms of facilitating bidirectional train movement. The third option presents a situation where one track is exclusively designated for freight and the other for passengers; this scenario may involve multiple tracks but does not qualify as a double track system focused on opposing traffic flows. The fourth choice mentions a single track designed for high-speed rail, which contradicts the concept of a double track system altogether, as it involves only one track rather than two.