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Alevel junction (or in theUnited Kingdom aflat crossing) is arailway junction that has a track configuration in which merging or crossing railroad lines provide track connections with each other that require trains to cross over in front of opposing trafficat grade (i.e. on the level).
The cross-over structure is sometimes called adiamond junction ordiamond crossing in reference to the diamond-shaped center. The two tracks need not necessarily be of the samegauge.A diamond crossing is also used as a component of adouble junction, like the one illustrated on the right.
The opposite of a level junction is aflying junction, where individual tracks rise or fall to pass over or under other tracks.
Conflicting routes must be controlled byinterlockedsignals to prevent collisions.
Level junctions, particularly those of fine angles or near right angles, createderailment risks and impose speed restrictions. The former can occur as the flanges of the wheels are momentarily unsupported and unguided and can slip through the gaps in the rails, and the latter because the assembly contains elements that can break or vibrate loose.
Level junctions are considered a maintenance issue by railroad companies as the inherent gaps tend to be hard on locomotive and rolling stock wheelsets.Switched diamonds partially solve these problems, but introduce new ones.
Flat crossings are particularly common in theUnited States where the lines of one company cross the lines of another company, and there is no particular need for the lines to be connected for through traffic.
Some examples of two tracks crossing another two tracks:
Flat crossings appear in some urban passenger rail systems, which can cause delays at peak hours as a train heading in one direction may have to wait for trains heading in another direction to clear the junction before it can cross. The junctions leading onto and off from the Loop of theChicago "L" are examples of this problem. TheNew York City Subway system mostly usesflying junctions, but in a few older parts of the system, flat crossings can still cause delays. Examples include the142nd Street Junction and theMyrtle Avenue junction.
Level junctions are often found ontram orstreetcar networks where lines cross or split. TheMBTA in Boston has two of these underground on theGreen Line, one where theE Line departs the central subway just west ofCopley Station, and another where theC Line andD Line split west ofKenmore Station. While the latter crossing rarely causes delays, the former is at an intersection of four lines and cars often have to wait for others to pass at peak hours. Earlier such splits in Boston (such as the disused crossing west ofBoylston Station) were built as flying junctions, but the two level splits were built as level junctions mostly to save money.
Adiamond crossing between1,435 mm (4 ft 8+1⁄2 in)standard gauge and1,600 mm (5 ft 3 in),broad gauge exists atArarat inVictoria,Australia[citation needed].
AtPorthmadog, in theUnited Kingdom, there is a flat-crossing between the single trackstandard gaugeCambrian Line and the narrow-gaugeWelsh Highland Railway (1 ft 11+1⁄2 in or597 mm, also single track).[2]
InDarby, Pennsylvania, USA, theSEPTA Route 11 line, usingPennsylvania trolley gauge of5 ft 2+1⁄2 in (1,588 mm), crossesCSX Transportation using standard gauge.[3]
InSouth Bay, San Diego at theSouth Bay Salt Works, as of 2001, survives the only crossing of narrow gauge track with standard gauge track, formerly utilized by theSan Diego and Arizona Eastern Railway, in the United States.[4]
InSofia,Bulgaria, there are two diamond crossings between tracks of theSofia tramway and a standard gauge railway line that connects Zaharna fabrika station with Zemlyanethermal power plant. One crossing is at Aleksandar Stamboliyski boulevard where the railway line crosses narrow gauge tram routes 8 and 10. The other is at Vazkresenie boulevard where it crosses tram routes 11 and 22. This second crossing is rather unique, as the tram track there isdual gauge. Route 11 uses narrow gauge while route 22 uses standard gauge. The railway line is rarely used, however. Most recently it saw use during the2009 Gas Crisis when Zemlyane TPP temporarily switched from burning natural gas to burningmazut, which was delivered to the power plant by train. In addition, the Sofia tramway has three diamond crossings between narrow and standard gauge tram tracks.
InQueensland,Australia, a number of flat crossings betweennarrow gauge (610 mm (2 ft)) cane tramways and main lines (1,067 mm (3 ft 6 in)) have been replaced bydrawbridges so that the rails of the main line are completely unbroken by gaps or weak spots: this allows the main line speeds to be raised.[5]
Media related toDiamond crossings at Wikimedia Commons