Minecraft wiki entry for Rail, associated text: An existing straight, north-south rail re-orients to east-west when a new rail is placed at the east or west sides.
If placed next to an existing rail that is one block up or down, the new rail slants up or down to join it. Rail "prefers", in order: west, east, south, and north. Other configurations can be created by placing and removing rails.
If a track is placed perpendicular to an existing length of track, it appears as a straight rail, but in fact, it is curved according to the patterns for tee junctions as seen above; mine carts going through the intersection turn the corner. Breaking and re-laying track so that the intersection block is laid last causes the intersection block to be updated as a curved section.