Flying junction

A flying junction or flyover is a railway junction at which one or more diverging or converging tracks in a multiple-track route cross other tracks on the route by bridge to avoid conflict with other train movements. A more technical term is "grade-separated junction". A burrowing junction or dive-under occurs where the diverging line passes below the main line.
The alternative to grade separation is a level junction or flat junction, where tracks cross at grade, and conflicting routes must be protected by interlocked signals.
Complexity.
Simple flying junctions may have a single track pass over or under other tracks to avoid conflict, while complex flying junctions may have an elaborate infrastructure to allow multiple routings without trains coming into conflict, in the manner of a highway stack interchange.
Flying junction without crossings.
Where two lines each of two tracks merge with a flying junction, they can become a four-track railway together. This happens regularly in the Netherlands (see Examples below).
High-speed rail.
Nearly all junctions with high-speed railways are grade-separated. On the French LGV high-speed network, the principal junction on the LGV Sud-Est, at Pasilly where the line to Dijon diverges, and on the LGV Atlantique at Courtalain where the line to Le Mans diverges, are fully grade separated with special high-speed switches ("points" in British terminology) that permit the normal line speed of on the main line, and a diverging speed of . 
The LGV (Lignes à Grande Vitesse) network has four grade-separated high-speed triangles: Fretin (near Lille), Coubert (south-east Paris), Claye-Souilly (north-east Paris) and Angles (Avignon). A fifth, Vémars (north-east Paris), is grade separated except for a single-track link on the least-used side, linking Paris Gare du Nord and Paris CDG airport.
Examples.
Finland
There are between 25 and about 40 flying junctions on Dutch railways, depending on how more complex examples are counted.
Flying junctions where the merged lines become a four track railway:
More complex flying junctions, with tracks from four directions joining: