Double junction

A double junction is a railway junction where a double track railway splits into two double track lines. Usually, one line is the main line and carries traffic through the junction at normal speed, while the other track is a branch line that carries traffic through the junction at reduced speed.

A number of configurations are possible.

The simplest and oldest arrangement consists of two turnouts and a fixed diamond crossing. Because the diamond needs to be relatively coarse, say 1 in 8, the curve radius is necessarily small, leading to a speed of perhaps 25 km/h. This type of junction is common on street-running tramways, where speeds are quite low and junction must fit into the available road space. Because the points are close together, the entire junction can be controlled by the mechanical point rodding of a single signal box.

Signal passed at danger (SPAD) protection — A train from R to P with 12 points reverse is protected from a train from P doing a SPAD by 11 points also lying reverse. A train from P to Q is NOT protected from a train from R doing a SPAD.

The fixed diamond can be replaced with a switched diamond, which eliminates the gap in rails at the K-crossing, which allows a higher speed if the geometry is poor. However, switched diamonds are not a perfect solution to the K-crossing problem, as the switches are very coarse compared to the finer switches of a turnout, and require high maintenance. The additional ends are also awkward to control unless power operated point machines are used.

An improved junction replaces the diamond with turnouts, which can be of as fine an angle as possible, so that this junction can carry branch traffic at high speed. This configuration assumes power operation of the points, as high speed turnouts are generally not suitable for mechanical operation. The high speed turnouts may require more than one point machine each. The turnouts can have no superelevation while the curve in the branch can; therefore the radius in the turnouts must be greater than the radius of the curve in the branch.

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Wikipedia