In U.S. railroad practice, a level junction (or in the United Kingdom a flat junction) 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. The structure is sometimes called a diamond junction or diamond crossing in reference to the diamond-shaped center. The two tracks need not be of the same gauge. The opposite of a level junction is a flying junction, where individual tracks rise or fall to pass over or under other tracks. A diamond crossing is also used as a component of a Double junction.
Risks
Routings must be controlled by signals and an interlocking plant, or by an automated Centralized Traffic Control (CTC) system to prevent collisions. 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 some new problems. If possible, diamonds should be replaced by a pair of turnouts.
Examples
Flat crossings are particularly common in the United 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. A notable example is Newark flat crossing, United Kingdom, which is on the East Coast Mainline, and the Lincoln to Nottingham line. It is certainly the fastest in the UK, with East Coast trains allowed to cross the junction at 100mph (160 km/h). Flat crossings also 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 headed in another direction to clear the junction before it can cross. The junctions leading onto and off from the Loop of the Chicago El are examples of this problem. The New York City Subway system uses mostly flying junctions, but in a few older parts of the system, flat crossings can still cause delays. Examples include the crossing in Upper Manhattan where the uptown (northbound) "3" line departs from the Lenox Avenue line and turns toward the west, crossing over the tracks of the downtown "2" line. 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. Diamond crossings between meter and broad gauge tracks are formed in Indore and Ratlam in India. At Rochelle Railroad Park, the double track UP mainline crosses the double track BNSF mainline forming four diamond crossings altogether at this location.
Drawbridge crossing
In Queensland, Australia flat crossings between narrow gauge cane tramways and main lines are being replaced by drawbridges 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.
