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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.
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 among a variety of tracks without trains coming into conflict, in the manner of a highway stack interchange.
Flying junction without crossings 
In some cases, when two lines of two tracks each merge with a flying junction, they become a four track railway together. In this case crossings of trains may be entirely avoided, or crossings may take place at a different place than at the junction, e.g. at a main railway station. This happens regularly in the Netherlands (see #Examples below).
High-speed rail 
Nearly all junctions leaving or joining high-speed railways are grade-separated. On the French TGV high-speed network, the principal junction on the LGV Sud-Est at Pasilly where the line to Dijon diverges from the line to Lyon, and the junction on the LGV Atlantique at Courtalain where the line to Le Mans diverges from the line to Tours, are both fully grade-separated junctions equipped with special high-speed switches (points in British terminology) which permit the normal linespeed of 300 km/h (186 mph) along the direction of the mainline, and a diverging speed of 220 km/h (137 mph).
The French LGV (Lignes à Grande Vitesse) network is large enough to contain four fully grade-separated high-speed triangles: Fretin (near Lille), Coubert (south-east Paris), Massy (south-west Paris) and Angles (Avignon). A fifth triangle, Vémars (north-east Paris) is grade-separated except for a single-track link on the least-commonly used side (southern end linking Paris Gare du Nord to Paris CDG airport).
- Bowen Hills railway station in Brisbane, Australia
- Burnley railway station in Melbourne, Australia
- Camberwell railway station in Melbourne, Australia
- Central Station in Sydney, Australia
- Sandgate Flyover, Newcastle - main line flies over coal branch line
- West Toronto Diamond in the Union Station Corridor
- France (LGV Triangles)
- Triangle de Fretin, Lille, France. Connecting Paris, Brussels and London. (map)
- Triangle de Coubert, Paris, France. (map)
- Triangle des Angles, Avignon, France. With two parallel 1.5 km (0.93 mi) viaducts. (map)
- "Triangle de Messy", Paris, France. Partial four-way junction. (map)
- Triangle de Vémars, Paris, France. (map).
There are between 25 and about 40 flying junctions on dutch national railways, depending on how one counts the more complex examples.
- Junction near Harmelen. Before conversion to a flying junction, this was the site of the Harmelen train disaster.
- Junction at Breukelen railway station
- Junction at Lage Zwaluwe railway station
Flying junctions where the merged lines become a four track railway:
- Junction near Den Haag Laan van NOI railway station
- Junction north of Leiden where lines from Haarlem and Schiphol merge
- Junction at Boxtel railway station where lines from 's-Hertogenbosch and Tilburg merge
- Junction west of Gouda where lines from Rotterdam and The Hague merge
More complex flying junctions, with tracks from four directions joining:
- Junctions around Amsterdam Sloterdijk railway station
- Junctions around Duivendrecht railway station
- Northwest exit of Utrecht Centraal railway station
- West and northwest exit of Rotterdam Centraal railway station
- Flying junctions at both sides of Weesp railway station (see diagram at right)
- United Kingdom
- Hamilton Square underground station, Birkenhead on the Merseyrail urban rail network.
- Aynho Junction in Aynho, Northamptonshire
- Worting Junction near Basingstoke, Hampshire (Note: the actual flyover (grade separation) is known as Battledown Flyover)
- Cogload Junction near Taunton
- Weaver Junction near Dutton, Cheshire (map) - the first ever
- Shortlands Junction in south London
- The junction northwest of Harrow-on-the-Hill, in the north London suburbs
- Hitchin flyover, under construction north of London
- Bleach Green Viaducts & Junction, Whiteabbey, Northern Ireland
- United States of America
- An abandoned underground flying junction exists on the Tremont Street Subway approaching the Pleasant Street Incline in Boston, Massachusetts.
- The two southern branches of the MBTA Red Line in Boston split via a flying junction just north of JFK/UMass station. In addition, lead tracks to Cabot Yard maintenance facilities branch off from the junction.
- Broad Street Subway in Philadelphia has three flying junctions to un-built spurs on Stenton Avenue, the Roosevelt Boulevard, and Passyunk Avenue.
- Flying junctions exist along the New York-Washington section of the Northeast Corridor, and the Philadelphia-Harrisburg section of the Keystone Corridor, both converging together at Zoo Interlocking near 30th Street Station in Philadelphia. All were built by the former Pennsylvania Railroad and are now maintained by Amtrak.
- The New York City Subway features many flying junctions across its system.
- Flying junction connects Metro-North Railroad's New Haven Line and Harlem Line, near Wakefield station in the Bronx.
- The Market Street Subway in San Francisco has a flying junction where streetcars from the J Church and N Judah lines join the main line of the subway. The subway portal is east of the intersection of Church Street and Duboce Avenue in the Duboce Triangle neighborhood, immediately north of a Safeway supermarket and south of the San Francisco branch of the United States Mint.
- The Regional Transportation District in Denver has a flying junction between the Southeast Corridor and the I-225 Corridor. At this location, the Southeast Corridor is located on the west side of I-25 and the I-225 Corridor is located in the median of I-225. The grade separations of the junction are woven into the grade separations of the interchange between the two highways. (map)
- All mainline connections on the Washington Metropolitan Area Transit Authority (Metro) rapid transit system use flying junctions. Adjacent to the Pepco power plant on Benning Road (near the Stadium-Armory station) is a large three track structure with a turnback pocket where the Blue and Orange lines meet. This would have been part of the Oklahoma Avenue station, had it been built. South of the King Street station in Alexandria is a series of tunnels where the Blue and Yellow lines meet. There are flying junctions at three underground rail stations: Rosslyn (Blue and Orange lines), L'Enfant Plaza (Green and Yellow lines), and the Pentagon (Blue and Yellow lines).
- Bloomfield (NJT station) to eliminate level crossings
See also 
- WikiMapia link - aerial photo of Fretin triangle (mentioned above)
- Photo link - flying junction on Pennsylvania Railroad north of 30th Street Station, Philadelphia, Pennsylvania, United States
- Photo link - simpler flying junction at terminus of Market-Frankford transit line, 69th Street Station, Philadelphia, Pennsylvania, United States