Track junctions are formed by the combination of points and crossings. Their main objective is to transfer rail vehicles from one track to another or to enable them to cross from one track to another. Depending upon the requirements of traffic, there can be several types of track junctions with simple track layouts. The most commonly used layouts are discussed in the following sections.
TURNOUT OF SIMILAR FLEXURE
A turnout of similar flexure (Fig.) continues to run in the same direction as the main line curve even after branching off from it. The degree of the turnout curve will be higher than that of the main line curve. The degree and radius of the turnout curve are given by the formulae:
|Turnout of similar flexure|
where Ds is the degree of the outer rail of the turnout curve from the straight track, Dm is the degree of the rail of the main track on which the crossing lies, i.e., the inner rail in Fig. above, Dt is the degree of the rail of the turnout curve on which the crossing lies, i.e., the outer rail, Rs is the radius of the outer rail of the turnout curve from the straight track, and .ft, is the radius of the rail of the turnout curve on which the crossing lies, i.e., the outer rail.
TURNOUT OF CONTRARY FLEXURE
A turnout of contrary flexure (Fig.) takes off towards the direction opposite to that of the main line curve. In this case, the degree and radius of the turnout curve are given by the following formulae:
Dt = Ds-Dm
Rt = Rs Rm /(Rm –Rs) Here, Dm is the degree of the rail of the main track on which the crossing lies, i.e., the outer rail in Fig. below.
|Turnout of contrary flexure|
“Alien a straight track splits up in two different directions with equal radii, the layout is known as a symmetrical split (Fig. below). In other words, a symmetrical spat is a contrary flexure in which the radii of the two curves are the same.
In a three-throw arrangement, two turnouts take off from the same point of a main line track. Three-throw switches are used in congested goods yards and at entry points to locomotive yards, where there is much limitation of space. A three-throw switch has two switches and each switch has two tongue rails placed side by side. There is a combined heel block for both the tongue rails of the switch. The switches can be operated in such a way that movement is possible in three different directions, that is, straight, to the right, and to the left. Three-throw switches are obsolete now as they may prove to be hazardous, particularly at higher speeds, because the use of double switches may lead to derailments.
|Three-throw switch (similar flexure)|
|Three-throw switch (contrary flexure)|
A double turnout or tandem is an improvement over a three-throw switch. In a double turnout, turnouts are staggered and take off from the main line at two different places. This eliminates the defects of a three-throw switch, as the heels of the two switches are kept at a certain distance from each other. The distance between the two sets of switches should be adequate to allow room for the usual throw of the point.
Double turnouts are mostly used in congested areas, particularly where traffic is heavy, so as to economize on space.
|Double turnout with similar flexure|
|Double turnout with contrary flexure|
A diamond crossing is provided when two tracks of either the same gauge or of different gauges cross each other. It consists of two acute crossings (A and C) and two obtuse crossings (B and D).
In a diamond crossing, the tracks cross each other, but the trains from either track cannot change track. Slips are provided to allow vehicles to change track. The slip arrangement can be either single slip or double slip. In single slips, there are two sets of joints, the vehicle from only one direction can change tracks. In the single slip shown in Fig. , the train on track A can change to track D, whereas the train on track C remains on the same track, continuing onto track D.
In the case of double slips, there are four sets of points, and trains from both directions can change tracks. In the double slip shown in Fig. the trains on both tracks A and C can move onto either track B or D.
A scissors crossover enables transferring a vehicle from one track to another track and vice versa. It is provided where lack of space does not permit the provision of two separate crossovers. It consists of four pairs of switches, six acute crossings, two obtuse crossings, check rails, etc.
Gauntletted track is a temporary diversion provided on a double-line track to allow one of the tracks to shift and pass through the other track. Both the tracks run together on the same sleepers. It proves to be a useful connection when one side of a bridge on a double-line section is required to be blocked for major repairs or rebuilding. The speciality of this layout is that there are two crossings at the ends and no switches [(a)].
|(a) Gauntletted track|
Gauntletted tracks are also used on sections where trains have to operate on mixed gauges, say, both BG and MG, for short stretches. In such cases, both the tracks are laid on the same set of wooden sleepers [(b)].
|(b) Gauntletted track for mixed gauge|
A double junction (Fig. below) is required when two or more main line tracks are running and other tracks are branching off from these main line tracks in the same direction. The layout of a double junction consists of ordinary turnouts with one or more diamond crossings depending upon the number of parallel tracks.
Double junctions may occur either on straight or curved main lines and the branch lines may also be either single or double lines. These types of junctions are quite common in congested yards.