This information applies to all track standards: ScaleSeven, O fine and O coarse. It is based on Midland Railway drawings c. 1875-1890, but is believed to be reasonably representative of main line railways of the time. Since trackwork lasted a long time, it is likely to apply to most of the 20th century also.

The basic geometry of a semi-curved switch is shown in Figure 1.


Figure 1. Geometry of a semi-curved switch.


Figure 2. Semi-curved A switch.

The switch is carried on 12 timbers, dimensions shown on the drawing. Offset dimensions are shown against each timber.

Switch angle: 1 in 13.24 or 4° 20'. To aid in drawing, the line tangential to the heel cuts the stock rail 102mm before the heel H or 38mm from the toe T.

Figure 3. Semi-curved B switch.

The switch is carried on 13 timbers, dimensions shown on the drawing. Offset dimensions are shown against each timber.

Switch angle: 1 in 17.84 or 3° 12'. To aid in drawing, the line tangential to the heel cuts stock rail 111.5mm before the heel H or 46mm from the toe T.

Figure 4. Semi-curved C switch.

Rail head width 1.6mm. All dimensions in mm. Sleepers 12in x 6in x 9ft basic dimensions. For 8ft 6in sleepers reduce all lengths by 3.5mm.

Switch curve drawn straight for simplicity. Values of h<sub>d<\sub> calculated for curve.

Figure 5. GWR bullhead curved B switch.

The switch is carried on 13 timbers - dimension (in mm) shown on the drawing. Offset dimensions are shown against each timber.

Switch angle - 1 in 15.77 or 3°38'. To aid in drawing - the line tangential to the heel cuts stock rail 110.5mm before the heel H or 47mm from the toe T.

Figure 6. Planing of curved switches.

This article was compiled by Michael Holland for the Gauge O Guild Manual and updated in July 2021. It was adapted for the GOGWiki by Nick Baines.