Passing Loops, also known as passing sidings in the USA, are stretches of track that engines can go on if a train is on the same line as another one heading in opposite directions. Four track parts of the main line are also long passing loops, however, do not include stations with an extra platform as those are bay platforms.
Usually, when engines like Thomas are on the same line as another engine on the same line coming from the opposite direction switch onto the passing loop when there are no points to switch them to the other line.