This article identifies and describes the component parts of a turnout and provides important dimensional information. More detailed information and dimensional data about particular components is contained in the following articles:

• Straight switches
• Semi-curved switches
• Fully curved switches
• Turnout curve tables (download)
• Common crossing
• Obtuse crossing
• Sleepers

The terms used in point and crossings are illustrated in Figure 1 and defined in the table below.


Figure 1. Geometry of a turnout.

Note in particular that there are two definitions that include the term lead. Throughout this work, the definition of lead, symbol L_T, is the distance between the switch heel and the intersection of the gauge lines. The other definition, symbol L, is the distance from the switch toe to the nose of the crossing and is referred to as the full lead. The omission of the word “full” can often cause confusion. The switch heel is the point about which the switch blade theoretically pivots; this may be a true pivot or merely a point at which the switch blade becomes free to bend. The location of the heel is determined by the amount of clearance required by wheels passing along the Stock Rail when the Switch Blade is open.

There were three types of traditional switch, although modern developments are creating variants. They are the straight switch, the semi-curved switch and the curved switch (Figure 2).


Figure 2. Types of turnout switches.

This type of switch was used by most of the pre-grouping companies. Three variants (see Figure 3) for pivoting it exist, of which the most popular was the tongue switch. Here the fishplate by which the switch blade is attached to the closure rail provides the pivot. In the heel switch, the blade meets its closure rail in a heel chair that permits it to pivot while preventing it from sliding out. This type was not in common use in bullhead track but is met in some FB track using a heel block.


Figure 3. Straight switch types.

The term “straight” refers to the appearance of the whole switch blade, and should not be confused with the term “straight cut” which defines the cross section of the planed area of the switch blade, see Figure 4.


Figure 4. End views of straight, undercut and chamfered switches. Note that the flat bottomed rail versions requires the rail foot to be machined to accommodate the switch. Of the three versions, modelers will find the straight cut version generally easier to reproduce.

The spring switch was a later development, in which the switch rail is secured in one or more chairs before the joint with the closure rail and therefore the heel length L_H is less than the switch rail length L_S. In some installations, the switch rail began to curve in these chairs and thus became an ancestor of the semi-curved switch. On main railways, the straight switch only existed in bullhead form, although there are flat bottom installations in industrial sidings and light railways.

Straight switch sizes are in heel lengths of 6, 9, 12, 15, 18, 24 and 30 feet, although the shorter ones are too rigid to be used as spring switches.

This type began to be used after 1923 by the LPTB and all the mainline companies except the GWR. The blade is straight for the length of the planing, after which it curves at the nominal radius of the turnout, to the point at which it is joined to its closure rail. These switches were always of the sprung type and so L_S exceeded the heel length, L_H by at least 2ft 6in. Semi-curved switches were designated A, B, C, D, E and F and largely superseded the 9, 12, 15, 18, 24 and 30 feet straight switches respectively. A flat bottom version exists in all but A size.

This was used by the GWR instead of the semi-curved switch. In this type the entire switch was curved from toe to heel. They were designated B, C and D, and corresponded to the semi-curved switches of the same designation. A, E and F sizes were not used. For its long leads the GWR used a 30ft straight switch. British Railways used all six sizes in flat bottom form, albeit with slightly different geometry. More complex forms with transition curvature and planing are in current use to accommodate the higher speeds of today.

This term, often confused with the straight switch does not refer to the switch geometry, but to the vertical profile of the switch planing as viewed looking on the toe (Figure 4). The switch rail is planed to a thickness of 0.375in. at the toe and, in order to avoid entry shock at a facing point, the stock rail is joggled also by 0.375in. A later development is to plane the switch toe to 0.75in and then chamfer the top 2.5in. down to 0.375in. Flat bottom rail has too much lateral stiffness to joggle and a housing is usually machined in the stock rail instead.

Another principal type of switch end profile is the undercut, in which the switch toe is planed down to a fine edge at the top, the 0.75in. thickness being attained lower down by the switch rail fitting under the head of the stock rail. More recently, both switch and stock rail are being chamfered to provide greater strength at the toe (Figure 4).

All three profiles are used for bullhead rail, but flat bottom switches are usually undercut or chamfered.

Generally, the straight cut profile is the easiest for the modeller, using only hand tools, to produce. Many joggles to house the switch, particularly in bullhead track, are overdone, thereby spoiling the appearance of the trackwork and some may prefer to file a tapered housing in the stock rail to correspond with the planing of the switch rail as for flat bottom track.

There are three types of turnout relevant to curved and semi-curved switches, namely natural, compound and natural plus straight:

• A natural turnout is one where the turnout radius R_T and switch radius R_S are equal.
• A compound turnout is one where R_T is less than R_S.
• A natural plus straight turnout has R_T equal to R_S, but has a length of straight inserted between the end of the turnout curve and the gauge intersection to increase the lead.

Despite what many modellers believe, in the majority of cases the turnout curve continued through the crossing. A straight crossing is easier for the model suppliers to produce and for the modeller to set out, but those who do attempt a curved crossing will be rewarded with track work that flows more readily.

Turnouts with curved and semi-curved switches are designated by combining the switch type with the crossing angle, i.e. B8 is a B switch and a 1 in 8 crossing, similarly A7, C12 etc.

For straight switches, there were several suitable crossing angles for each switch, typical examples being shown at Figure 8 in Prototype pointwork. Although this represents North British Railway track, that of other companies was not significantly different. Even so, those wishing to model a specific prototype and period are recommended to study works covering the subject in greater detail than is possible here (see Source of information, below).

With the exception of ScaleSeven, where the only compromises are due to minor differences in railhead width compared with the prototype, there are significant geometrical variations between model and prototype. Depending on the standards adopted these prevent strict adherence to scaled down versions of full sized pointwork.

Both fine and coarse standards are under scale gauge by about 1mm. As a result, a turnout curve equating to a prototypical radius will generate an incorrect crossing angle and to maintain the ‘correct’ radius, the crossing angle would have to be altered. It is more practical to use the prototypical angle and change the turnout radius and this is the recommended method upon which the accompanying calculations and tables have been based.

The straight switch presents particular difficulties. Its prototype heel clearance, h_c, cannot be less than flangeway clearance which is, prototypically, 1¾in. This equates to 1.02mm in ScaleSeven but for fine standard the value has to be increased to 1.75mm and for coarse standard to 2.2mm (see Standards). This increases the heel divergence and changes the switch angle, spring of the turnout curve and turnout radius for a given crossing angle. The effect is increased further when the wider code 200 bullhead or code 220 flat-bottomed rail is used for coarse standard trackwork.

To illustrate the problem, in ScaleSeven, a 6ft straight switch deflects an approaching vehicle though 3.6° (1 in 16) at the toe, while in fine scale the deflection is 4.6° (1 in 12.5). For coarse scale with a railhead width of 1.6mm the angle is 5.2° (1 in 11) and this rises to 6.2° (1 in 9) if the 2.35mm wide code 200/220 rail is used. This would have such an effect both on smooth running through the turnout and on its appearance that some straight switches cannot be recommended although data has been given for those who wish to use them.

It is significant that ready-made pointwork supplied in earlier times for coarse scale and using the heavy section rail, approximated to fully curved switches, thereby giving a smoother run through the formation. For curved and semi-curved switches, the heel clearance exceeds the minimum flangeway for both fine and coarse scale where a rail head width of 1.6mm is used, and this can be used satisfactorily, but B semi-curved and GWR fully curved bull head switches can give problems of flangeway clearance when 2.35mm wide rail is used due to its greater rigidity.

Restrictions on space generally prevent most modellers from using all but the shortest switches and crossing angles. (To keep the data sheet tables within bounds the longest turnout listed is a B8, which has a closure rail radius of about 4270mm or 14ft). Once the basic design of a layout has been completed the types of turnouts required can be determined. This can be done by measuring the crossing angle and selecting a suitable pair of switches from the range suggested in Table 1. Table 2 covers the range of switches in greater detail.

Table 1. Recommended blade and crossing angle combinations.

¹ Assuming either code 124 bullhead rail or code 143 flat bottomed rail, having a rail head width (hr) of 1.6mm.

² Assuming either code 200 bullhead rail or code 220 flat bottomed rail, having a rail head width (hr) of 2.35mm.

³ These switches have a significant angle of deflection at the toe and should only be used where limited space prevents the use of longer turnouts.

† See text about clearance problems with B switches.

Table 2. Switch lengths and heel divergence details.

¹ Although data in this table have been included, straight switches are not recommended for the track standards in italics in other than exceptional circumstances because they involve significant angular deflection at the toe. Any such use should be restricted to sidings traversed at low speed.

² Southern Railway semi-curved bull head switches of A and B size differ slightly in that the heel divergence, H_d, is measured at the last chair instead of the end of the switch rail, i.e. 1ft 1in. before the joint with the closure rail. Consequently the values for heel divergence and switch angle are slightly different. The switch rail lengths are still 20ft (140mm) and 22ft 6in. (157.5mm) respectively but the measured heel length, L_H, is shorter by 7.6mm, while the lead LT is increased by the same amount.

³ The B semi-curved bullhead and B fully curved GWR switches cannot be recommended in 0 Coarse with rail head 2.35mm, i.e. Code 200 rail, where the rigidity of the rail is likely to lead to inadequate clearance between switch rail and stock rail at the pinch point.

The information in this article can assist in choosing the preferred switch type and length. Additional information and drawings are given in the articles on Straight, Semi-curved and Fully curved switches.

The suitability of the chosen combination of switch and crossing angle can be checked with the turnout curve tables. These tables give the overall length and turnout (closure rail) radius, and all other necessary dimensions. The dimensions from these tables can then be used to produce drawings as described in Making pointwork drawings.

Dimensions of the common crossing for the chosen crossing angle can be determined. For diamond crossings, slip points and the like, the dimensions of obtuse crossings are available.

Layouts of sleepers and chairs for turnouts, diamond crossings and slips are shown here.

Information on prototype practice:

A Century of Permanent Way, Bland (1927)

Railway Theory and Practice, Hepworth & Lee (1922)

GWR Standard Permanent Way Practice, Bowler (1923)

GWR Switch and Crossing Practice, David L. Smith (2002)

Check Rails & Chairs, LNER Chief Engineers Dept (1945)

British Railways Track (1st Edition), Permanent Way Inst. (1943)

British Railways Track (3rd Edition), Permanent Way Inst. (1964)

British Railways Track (6th Edition), Permanent Way Inst. (1993)

British Railways Track. Flat Bottom Track Supplement, Heller (1951)

Track Design Manual, British Railways Board (1974). (Now incorporated in the Industry Standards System)

BS 4521: 1971 Specification for railway turnouts for private users - Part 1: Turnouts over which British Railways locomotives operate - Section 1.1: Turnouts using BS9 section 95R bullhead rail - Section 1.2: Turnouts using BS11 section 110A flat bottom rail. Contains many drawings of turnouts and turnout components. Later versions of this British Standard may omit Section 1.1.

For the best pictures of real pointwork, with some complex combinations, from 1900 to 1940:

Great Eastern In Town and Country I, Hawkins (1990)

Great Eastern in Town and Country II, Hawkins (1991)

Great Eastern in Town and Country III, Hawkins (1997)

A guide intended for 4mm scale but with much information suitable for all modelling scales:

An Approach to Building Finescale Track in 4mm, Rice (1991)

Useful articles have appeared in magazines dealing with the prototype for which no easily accessible general index is available. An internet search, or a visit to the National Railway Museum will usually be helpful to those hunting for precise information. For modellers of the contemporary scene, an on-site visit may be possible, and for historical modellers, preservation railways often have extensive trackwork from earlier times (and are usually welcoming of visitors).

This article was compiled by M. Holland for the Gauge O Guild Manual. It was adapted for the GOGWiki by Nick Baines.