Referring to a section of track that has trains travelling in both directions.

In rural areas where the amount of rail traffic is lower than metropolitan areas, often there is only one track between two stations. A train may travel to the terminus and return along the same track (bi-directional). If multiple trains use the single track, the trains need to pass each other at stations or at passing loops. 

In metropolitan areas, it is common for two lines to be used. But two lines does not necessarily mean that one is used exclusively for the up-line traffic and one is used exclusively for the down line traffic. To permit timetable efficiencies, some tracks have been designed for bi-directional traffic during peak operations. This permits express trains for example to pass trains that are stopping at all stations.

A common track layout is three tracks. The two outer tracks may handle the normal train operations and the third track (or road) is used to speed other trains to their destinations. For example, this would be the outer suburbs to begin their passenger run in to town in the morning peak, or to speed the train to the main central station to begin their passenger run in the evening peak. The two outer tracks could still also be used for bi-directional traffic, adding to the complexity of options available.

Never assume that any track has trains travelling in one direction only!