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 300 km/h on the main line, and a diverging speed of 220 km/h.

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
Denmark
 * Junction of M1 and M2 lines on the Copenhagen Metro


 * France (LGV Triangles)
 * Triangle de Fretin, Lille, connecting Paris, Brussels and London. (map)
 * Triangle de Coubert, Paris. (map)
 * Triangle des Angles, Avignon, with two parallel 1.5 km viaducts. (map)
 * Triangle de Claye-Souilly, Paris, partial four-way junction. (map)
 * Triangle de Vémars, Paris. (map).
 * Hong Kong

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:
 * Where Airport Express and Tung Chung Line diverge from each other at Tai Ho Wan
 * Tseung Kwan O Line to the east of Tseung Kwan O Station
 * Netherlands
 * near Harmelen. Before conversion to a flying junction, this was the site of the Harmelen train disaster.
 * at Breukelen railway station
 * at Lage Zwaluwe railway station
 * near Den Haag Laan van NOI railway station
 * north of Leiden where lines from Haarlem and Schiphol merge
 * at Boxtel railway station where lines from 's-Hertogenbosch and Tilburg merge
 * west of Gouda where lines from Rotterdam and The Hague merge
 * around Amsterdam Sloterdijk railway station
 * around Duivendrecht railway station
 * North-west exit of Utrecht Centraal railway station
 * West and north-west exit of Rotterdam Centraal railway station
 * at both sides of Weesp railway station (see diagram at right)


 * Taiwan
 * Start of Shalun Line, south of Zhongzhou Station
 * After Hainault Station, it diverges separately into Daniel Line and Jeremy Line


 * Timothy North
 * United Kingdom
 * Pelaw Junction where both the Tyne and Wear Metro green line to South Hylton joins the Durham Coast Line and yellow line continues to  South Shields - both diverging on the bridge itself (map)
 * Springhead Junction on the North Kent Line
 * Southfleet Junction on the HS1
 * Norton Bridge Junction near Stone, Staffordshire
 * Hamilton Square underground station, Birkenhead, on Merseyrail
 * Aynho Junction in Aynho, Northamptonshire
 * Worting Junction near Basingstoke, Hampshire (the flyover is called Battledown Flyover)
 * Cogload Junction near Taunton
 * Weaver Junction near Dutton, Cheshire (map) - the first ever
 * Shortlands Junction in south London
 * North-west of Harrow-on-the-Hill, in the north London suburbs
 * Hitchin flyover, north of London
 * Reading West Junction
 * Bleach Green Viaducts & Junction, Whiteabbey, Northern Ireland


 * United States
 * Amtrak
 * Along the New York-Washington section of the Northeast Corridor, and on the Philadelphia-Harrisburg section of the Keystone Corridor, converging at Zoo Junction near 30th Street Station in Philadelphia. All were built by the former Pennsylvania Railroad and are now maintained by Amtrak. 39.97086°N, -75.19848°W
 * Boston, Massachusetts
 * An abandoned underground junction on the Tremont Street Subway approaching the Pleasant Street Incline
 * 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.
 * Chicago, Illinois
 * On the Chicago 'L', where Orange Line trains diverge from Green Line trains north of 18th Street, as well as underground where a non-revenue flying junction separates Red Line trains heading to 95th from those heading to the South Side main line, currently used to send some rush-period Red Line trains to Ashland/63rd. Another flying junction is planned to be built just north of Belmont/Sheffield to increase capacity on the Red Line, Brown Line, and Purple Line Express.
 * Denver, Colorado
 * On the Regional Transportation District in Denver between the Southeast Corridor and the I-225 Corridor: the Southeast Corridor is on the west side of I-25 and the I-225 Corridor is 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)
 * New York, New York
 * On the New York City Subway there is an above-ground example at Hammel's Wye on the IND Rockaway Line (map), as well as numerous below-ground examples across the network.
 * Connecting Metro-North Railroad's New Haven Line and Harlem Line, near Wakefield station in the Bronx.
 * Philadelphia, Pennsylvania
 * Amtrak's Zoo Junction is where the Northeast Corridor meets the Keystone Corridor. Also known as Zoo Interlocking, the name comes from the Philadelphia Zoo, which is located in the crescent shaped pocket between the junction and the river.
 * On the Broad Street Subway to un-built spurs on Stenton Avenue, the Roosevelt Boulevard, and Passyunk Avenue.
 * San Francisco Bay Area, California
 * The Oakland Wye, where all of Bay Area Rapid Transit's mainline operations converge near downtown Oakland.
 * On the Market Street Subway in San Francisco where 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.
 * Washington, District of Columbia
 * All main-line connections on the Washington Metro - 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, Silver 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 also flying junctions near three underground rail stations: Rosslyn (Blue, Silver, and Orange Lines), L'Enfant Plaza (Green and Yellow lines), and the Pentagon (Blue and Yellow lines).