Level junction

Not to be confused with Level crossing.

Flat junction: trains have to wait to cross the 'diamond' at the centre

A schematic diagram of a dual-gauge diamond crossing

Several diamond crossings (now obsolete after removal of one track direction) in the Port of Mainz, Germany

A fully assembled level junction used where the Union Pacific and Kansas & Oklahoma tracks cross

In U.S. railroad practice, 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 centre. The two tracks need not necessarily be of the same 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.

Risks

Conflicting routes must be controlled by interlocked signals 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 new ones.

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. At Rochelle Railroad Park, the double track UP mainline crosses the double track BNSF mainline forming four diamond crossings altogether at this location.

In the United Kingdom, a notable example is Newark flat crossing where the East Coast Mainline and Lincoln to Nottingham line cross (both double track). This is the fastest flat-crossing UK, with trains travelling north-south allowed to cross the junction at 100 mph (160 km/h). Protection of the junction is controlled by signalling.

Local transport

Chicago Transit Authority control tower 18 level junction

A pair of level junction interlaced turnouts at Chicago Transit Authority control tower 18 on the elevated Chicago 'L' north and southbound Purple and Brown lines intersecting with east and westbound Pink and Green lines and the looping Orange line above the Wells and Lake street intersection in the loop.

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 the Chicago 'L' are examples of this problem. The New York City Subway system mostly uses flying junctions, but in a few older parts of the system, flat crossings can still cause delays. Examples include the 142nd Street Junction and the Myrtle Avenue junction.

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.

Different gauges

Diamond crossings between meter and broad gauge tracks are formed in Indore and Ratlam in India.

At Porthmadog, in Wales, there is a flat-crossing between the single track standard gauge Cambrian Line and the narrow-gauge Welsh Highland Railway (1 ft 11+1⁄2 in (597 mm), also single track).

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.

External links

Media related to Diamond crossings at Wikimedia Commons