BY: Sagar Pokharel

Definition

A railroad switch or turnout is a mechanical installation which enables railway trains to be guided from one track to another, such as at a railway junction.

Combination of stock rail and tongue rail form a switch.

A set of switches is known as points.

The combination of points and crossing is called a Turn out

A turnout generally has a straight "through" track (such as the main-line) and a diverging route.

Right-hand switches have a diverging path to the right of the straight track, when coming from the narrow end, and a left-handed switch has the diverging track to the left of the straight track.

Here the Rail track A divides into two:-

Track B (the straight track) and Track C (the diverging track)

Various component parts of switches

Stock rail They are the main rails of the track to which the tongue

rails are fit closely

They are the outer rail in the turnout.

Tongue rail (Switch rail) Rails which lie between the 2 stock rails

End portion of the tongue rail is called toe and is connected by stretcher bar .

It is the moving part of the switch which diverts the train from one track to the other.

Requirements of tongue rails• Top and side of the tongue rail is tapered in such a way

that they do not bear any load

• Tongue rail is higher than stock rail by 6mm

• Half thickness of the tongue rail at the toe should be closely fitted within the stock rail

Stretcher bar Used to connect the toe of the tongue rails so that both

the tongues moves through the same distance or gap

Generally 2 or 3 bars are used near and behind the toe

Heel blocks or distance blocks These are the blocks inserted between the heel of the

stock rail and tongue rail

Used to provide clear gap for the wheel flange.

Sliding plate These are the special plates which are provided for

supporting and sliding the tongue rails. It is essential because it helps the tongue rail to move

toward and away from stock rails and tongue rails are able to slide.

Check rails They are the rails which are used guide the outer wheel

flange of the train. To ensure that the train does not derail.

Wing rail Rails which are used to guide the inner wheel flange of

the train.

Crossing

It is a arrangement of rails introduced at the junction where two rails cross to permit the wheel flange of a railway vehicle to pass from one track to another track.

Important Terms in connection with switches

Heel clearance / Heel divergence It is the distance between the running face of the stock rail and

gauge face of the tongue rail measured at the heel of the switch.

Heel clearance according to Indian standards for different gauges:

B.G Track:- 13.3cm to 13.7cm

M.G Track :- 11.7cm to 12.1cm

N.G Track:- 9.8cm

Flangeway clearance It is the distance between the adjacent faces of the

stock rail and the check rail.

It is provided as a clearance for the free movement of the wheel flange

Flangeway should take into account the amount of wear. It is observed that upto 6cm flangeway clearance, wear takes place.

Indian standards for the value of Flangeway clearance on the basis of wear are:

For 1 in 12 crossing angle, F.C = 6.3cm

For 1 in 8 ½ crossing angle, F.C = 6.6cm

Switch angle It is the angle between the Running face(Gauge face) of

the stock rail and tongue rail.

For smooth entry and movement of trains, it is desirable to have small switch angles on tracks used by fast moving trains.

Throw of Switch It is the distance through which the toe of the tongue

rail moves sideways to provide desired direction over the turnout.

Types of Switches

1. Stub switch

First developed for steam railways, was one in which the straight and diverging tracks were completely separate and side by side.

The throw of the switch was about 5 inches.

No separate tongue rail was provided.

This type is no more in use and has been replaced by Split Switch.

2. Split SwitchIn this type of switch a tongue rail is combined with

the stock rail.

Split Switches are classified as:

A) On the basis of fixation at heel:

(i) Loose Heel Type- In this type, tongue rails are joined to lead rails by means of

fish plates.

- Two front bolts are kept loose to allow the throw of the switch and these bolts are kept tight when the tongue is open

- This is suitable for short length switches.

(ii) Fixed Heel Type (Flexible Type)

- This is an improvement over loose heel type switch.

- In this all the 4 bolts are tight when the tongue is closed

- Gives satisfactory results when long tongue are used

- It is suitable for long tongue rails only.

B)

(i) Under Cut Switches

- In case the height of the stock and tongue rail is same, it is desirable to cut out a portion of flange at the foot of the stock rail so that toe of tongue rail is accommodated under head of the stock rail.

- The disadvantage of this type of switch is that it becomes weak because flange portion is cut out.

- These switch are generally used in narrow gauge lines.

ii) Over riding Switches:

- In this type separate rail sections of stock rail and tongue rail are adopted.

- The stock rail of heavy section and tongue rail of light section are used instead of cutting the flange rather than weakening the stock rail as in case of undercut switch.

- The tongue rail rides over the flange of the stock rail.

- Generally used in B.G and M.G track.

iii) Straight cut Switches:

- In this type the tongue rail is cut straight in the line with the stock rail.

- This is done to increase the thickness of toe of the tongue rail, which increases its strength

- This type of switch is suitable for Bull Headed rails.

Thank You