[openrailwaymap] Track bidirectional or not ?

Hello I have read https://wiki.openstreetmap.org/wiki/OpenRailwayMap/Tagging but I'm a bit confused about how to know if a track can be used in both directions. For a given way, should I rely on the value of the key oneway and on the value of the key railway:bidirectional ? And I'm confused about the description of the value "possible" for the key railway:bidirectional. Does it mean a way with that value may be bidirectional or not ? Thanks for your answers.