Turnouts

The simplest arrangement of points and crossing can be found on a turnout taking off from a straight track. There are two standard methods prevalent for designing a turnout. These are the (a) Coles method and the (b) IRS method.

These methods are described in detail in the following sections.

The important terms used in describing the design of turnouts are defined as follows.

Curve lead (CL) This is the distance from the tangent point (T) to the theoretical nose of crossing (TNC) measured along the length of the main track.

Switch lead (SL) This is the distance from the tangent point (T) to the heel of the switch (TL) measured along the length of the main track.

Lead of crossing (L) This is the distance measured along the length of the main track as follows:

Lead of crossing (L) = curve lead (CL) - switch lead (SL)

Gauge (G) This is the gauge of the track.

Heel divergence (D) This is the distance between the main line and the turnout side at the heel.

Angle of crossing (a) This is the angle between the main line and the tangent of the turnout line.

Radius of turnout (R) This is the radius of the turnout. It may be clarified that the radius of the turnout is equal to the radius of the centre line of the turnout (R1) plus half the gauge width.

R = R1 + 0.5G

As the radius of a curve is quite large, for practical purposes, R may be taken to be equal to R1.

Special fittings with turnouts

Some of the special fittings required for use with turnouts are enumerated below.

Distance blocks Special types of distance blocks with fishing fit surfaces are provided at the nose of the crossing to prevent any vertical movement between the wing rail and the nose of the crossing.

Flat bearing plates As turnouts do not have any cant, flat bearing plates are provided under the sleepers.

Spherical washers These are special types of washers and consist of two pieces with a spherical point of contact between them. This permits the two surfaces to lie at any angle to each other. These washers are used for connecting two surfaces that are not parallel to one another. Normally, tapered washers are necessary for connecting such surfaces. Spherical washers can adjust to the uneven bearings of the head or nut of a bolt and so are used on all bolts in the heel and the distance blocks behind the heel on the left-hand side of the track.

Slide chairs These are provided under tongue rails to allow them to move laterally. These are different for ordinary switches and overriding switches.

Grade off chairs These are special chairs provided behind the heel of the switches to give a suitable ramp to the tongue rail, which is raised by 6 mm at the heel.

Gauge tie plates These are provided over the sleepers directly under the toe of the switches, and under the nose of the crossing to ensure proper gauge at these locations.

Stretcher bars These are provided to maintain the two tongue rails at an exact distance.

Coles method

This is a method used for designing a turnout taking off from a straight track (Fig. 14.11). The curvature begins from a point on the straight main track ahead of the toe of the switch at the theoretical toe of switch (TTS) and ends at the theoretical nose of crossing (TNC). The heel of the switch is located at the point where the offset of the curve is equal to the heel divergence. Theoretically, there would be no kinks in this layout, had the tongue rail been curved as also the wing rail up to the TNC. Since tongue rails and wing rails are not curved generally, there are the following three kinks in this layout.

Turnout from a straight track (Coles method)
Fig. 14.11 Turnout from a straight track (Coles method)

(a) The first kink is formed at the actual toe of the switch.

(b) The second kink is formed at the heel of the switch.

(c) The third kink is formed at the first distance block of the crossing. The notations used in Fig. 14.11 are the following.

Curve lead (CL) = AE = TE'

Switch lead (SL) = TL

IRS method

In this layout (Fig. 14.12), the curve begins from the heel of the switch and ends at the toe of the crossing, which is at the centre of the first distance block. The crossing is straight and no kink is experienced at this point. The only kink occurs at the toe of the switch. This is the standard layout used on Indian Railways. The calculations involved in this method are somewhat complicated and hence this method is used only when precision is required.

Turnout from a straight track (IRS method)
Fig. 14.12 Turnout from a straight track (IRS method)

Example 14.3 A turnout is to be laid off a straight BG track with a 1 in 12 crossing. Determine the lead and radius of the turnout with the help of the following data: heel divergence (d) = 133 mm, crossing angle (a) = 4° 45'49", switch angle

(b) = 1° 8'00", straight length between the theoretical nose of crossing and the tangent point of crossing (h) = 1.418 m.

Solution

a = 4° 45'49", b = 1° 8'0"

G = 1.676 m, d = 0.133 m N = 12, h = 1.418 m

Standard turnouts and permissible speeds

On Indian Railways, normally 1 in 8.5 turnouts are used for goods trains while 1 in 12 and 1 in 16 turnouts are used for passenger trains. Recently 1 in 20 and 1 in 24 turnouts have also been designed by the RDSO, to be used to permit higher speeds for fast trains on the turnout side. The maximum speeds permitted on these turnouts are given in Table 14.5.

Table 14.5 Permissible speeds on turnouts

Gauge

Type of

turnout

Switch angle

Permissible speed (km/h)

BG

1 in 8.5

1° 34' 27"

10* for straight switch and 15 for curved switch for 52/60 kg on PSC sleepers

BG*

1 in 8.5

Symmetrical split (SS) 0° 27' 35''

30 for curved switch as well as SS with 52/60 kg on PSC sleepers; 15* for curved switch for 52/60 kg on PSC sleepers*

BG

1 in 16

1° 8'0'' 0° 24'27''

50 or 6011

MG

1 in 8.5

1° 35'30'' 0° 29' 14''

10 for straight as well as curved switch

MG

1 in 12

1° 09'38'' 0° 24'27''

15 for straight switch and 15 for partly curved switch

MG

1 in 16

0° 24'27''

30

Source: Indian Railway Permanent Way Manual (IRPWM)-Correction slip no. 94 dated 1 June 2004.

* As per the Indian Railway way and works (IRWW) manual, a speed of 15 km/h was originally permitted on 1 in 8.5 turnouts. However, due to a subsequent number of derailments of passenger trains on turn-in curves, the speeds on these turnouts have row been reduced to 10 km/h only.

* The figures in the second row correspond to curved switches.

* A speed of 30 km/h is also permitted on 1 in 12 turnouts on those interlocked sections where all turnouts over which a running train may pass are 1 in 12 throughout the section and the locomotives are fitted with speedometers. In all other cases, speed is restricted to 15 km/h only.

' 60 km/h permitted only for high-speed turnouts to Drg. No. RDST/T-403.

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