## The Double Slip Switch

This was often called a "puzzle switch" in the U.S.

A double slip switch is an acute-angle crossing with four pairs of points making connections within the crossings. The reader should consult a good photograph to understand the layout. On the Great Western Railway, they were known as double compound points, and the connections were slip roads. A train approaching on any of the four tracks may leave on either of the two tracks on the other side of the crossing. The saving in space is considerable, when compared to a crossing with two connecting tracks and four ordinary turnouts. These switches are generally found in stations, where space is at a premium. A simpler version has only two sets of points, and is called a single slip switch. In this arrangment, only trains entering on two of the tracks may choose their routes. The two middle crossings must be self-guarded or movable-nose, since there is no room for guard rails. The two end crossings may be normal ones, with guard rails. The double slip switch occupies no more room than the crossing would itself.

Four switch stands or switch motors could be required, but it is possible to connect the points so that only two are required. Indeed, if the pairs of points at each end are connected by a mechanism giving opposite movements, such as a T-lever, only one control is required. In one setting, the direct routes are selected, while in the other the diverging routes are. This I'll call the English connection, since it was the first used, in the 1880's, in the double slip switches supplied to Continental railways. This connection is illustrated in the diagram at the right. The small arrows show the positions of the points. In manual signalling, this is a very heavy pull, with eight points on one lever.

A simpler connection with fewer parts and less lost motion connects the four points at each end so that they move together. Two controls are now required, so there are four possibilities to select the four routes. This is illustrated in the diagram at the left. The indicator shown is the Cauer, introduced by the Deutsche Reichsbahn, and is the one now in use there and in Austria. Note that when one straight-through route is selected, the other cannot be used. It is clear that two routes can never be simultaneously used in any case. In the United States, two ordinary switch stands were used, and so the aspects of their targets could not be easily correlated with the route set.

Slip switches are used in station throats and yards, by trains moving at moderate or slow speeds, and seldom in main tracks. They are particularly useful for a diagonal track crossing others, allowing access to each track crossed. Other space-saving trackwork includes scissor crossovers and three-way switches.

The design of a slip switch is illustrated at the right. The frog angle F determines the distance AD, within which half of the slip track must fit. A No. 8 frog gives F = 7.153° and so AD = 37.8 ft for standard gauge, g = 4.7083 ft. The length L of the switch rail is then chosen, which determines S, the heel distance being standard. The point of the switch rail lies a distance m from the point of the frog. An 11-ft. switch rail, and a clearance distance m of 5 ft, leaves a distance 21.7 ft for half the closure rail. This distance is found as shown, first subtracting m, then finding the side BE of the triangle DBE (neglecting the point thickness), and finally subtracting L. BE is usually not much different from BD. Now the radius of the closure rail can be found, which is 1437 ft. in this case. We see that a slip switch is practical for a No. 8 frog. When the frog angle is small enough that F = 2S, the closure rail becomes straight. To construct the slip switch, the locations of the frog points are marked. Then the switch rails can be located and installed, and finally the closure rail can be laid. The switch and closure rails for the other side have not been shown, but are found the same way.

### References

W. G. Chapman, Track Topics (London: Great Western Railway, 1935. Reprinted by Patrick Stephens Ltd., 1971). An excellent reference for classic British railway engineering. The photograph on the title page shows a double compound, with two single compounds behind it, and then another double compound in the background. There are diagrams on p. 130. On p. 55 a photo of Reading old station in 1895 shows two strange single compounds with single-point switches.

C. F. Allen, Railway Curves and Earthwork, 6th ed. (New York: McGraw-Hill, 1920). Art. 178, p. 109.