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A level junction (or in the United Kingdom a flat crossing) is a railway 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 traffic at grade (i.e. on the level).
The cross-over structure is sometimes called a diamond junction or diamond crossing in reference to the diamond-shaped center. The two tracks need not necessarily be of the same Rail gauge. A diamond crossing is also used as a component of a double junction, like the one illustrated on the right.
The opposite of a level junction is a flying junction, where individual tracks rise or fall to pass over or under other tracks.
Level junctions, particularly those of fine angles or near right angles, create derailment 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.
Some examples of two tracks crossing another two tracks:
Level junctions are often found on tram or streetcar networks where lines cross or split. The MBTA in Boston has two of these underground on the Green Line, one where the E Line departs the central subway just west of Copley Station, and another where the C Line and D Line split west of Kenmore 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 of Boylston Station) were built as flying junctions, but the two level splits were built as level junctions mostly to save money.
At Porthmadog, in the United Kingdom, there is a flat-crossing between the single track standard gauge Cambrian Line and the narrow-gauge Welsh Highland Railway (, also single track).Glyn Williams, Tourist and Enthusiast Railways, Wales. Retrieved 2008-03-21.
In Darby, Pennsylvania, USA, the SEPTA Route 11 line, using Pennsylvania trolley gauge of , crosses CSX Transportation using standard gauge.
In South Bay, San Diego at the South Bay Salt Works, as of 2001, survives the only crossing of narrow gauge track with standard gauge track, formerly utilized by the San Diego and Arizona Eastern Railway, in the United States.
In Sofia, Bulgaria, there are two diamond crossings between tracks of the Sofia tramway and a standard gauge railway line that connects Zaharna fabrika station with Zemlyane thermal 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 is dual 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 the 2009 Gas Crisis when Zemlyane TPP temporarily switched from burning natural gas to burning mazut, 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.
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