Line equipped with two tracks so that each track is specialized for a single direction and the trains normally run along the left track, called legal track. In intermediate stations it is possible to create precedence. In certain cases, such as, for example, carrying out works in line on one of the two tracks, it is possible to use the track left in operation in both directions, in this case the circulation can take place according to different procedures, depending on the presence or absence of signaling systems on the adjacent track, for the opposite direction to the conventional direction of travel
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