Railroad switch

A railroad switch (in North America), or (set of) points (in Britain and Australia) — in technical usage, also a turnout — is a mechanical installation enabling trains to be guided from one line of rail tracks or tramway tracks to another.

In the illustration on the right, rail track "A" divides into two tracks, "B" and "C". Each switch contains a pair of linked tapering rails (point blades) that can be moved laterally into one of two positions, determining whether a train coming from "A" will be led towards "B" or towards "C." A train coming from "B" or "C" will be led to "A" anyway if the moving parts of the switch are not locked (passage in this direction through a switch is known as a "trailing movement"). Given, however, the potential for derailment at all but the slowest speeds in thus "forcing" a switch, and the fact that the switch blades on all main running-lines are mechanically or electrically locked into position, it is normal to set switches in the appropriate position for trailing movements. End "A" is referred to as the facing-point end.

Background

On early lines, vehicles were moved between tracks by means of sliding rails. The switch as we know it was patented by Charles Fox in 1832. The position of the switch is usually changed electrically on main lines and controlled from a remote control center, or an interlocking tower/signal box, from where staff also alter semaphores or light signals correspondingly. In rarely-used sidings, low-traffic branch lines, self-contained marshalling yards, or on heritage railways, a switch might be manually operated with a points lever. The switch points of tram lines are often operated remotely by the driver.

Prior to the widespread availability of electricity, switches at heavily-traveled junctions were operated from a switch tower/signal box constructed near the tracks through an elaborate system of rods and levers. With a right switch, "A" and "B" form a straight track and "C" is to the right of "B," with a left switch "C" is to the left. A switch may also be symmetrical, or tracks "AB" and "AC" may be curved at different radii in the same or different directions.

The correct setting of points is fundamental to the safe running of a railway. A fatal train accident at Potters Bar, Hertfordshire, UK occurred in May 2002, when a switch sprang to a different position as a coach crossed it. The front coach wheels therefore progressed from "A" to "B" whereas the rear wheels slewed towards "C," causing the whole coach to detach from the train and slew sideways across the platform ahead. Fortunately, the movement of the switch occurred beneath the final coach, so that although 7 people were killed, the front coaches remained on the tracks. Poor maintenance of the points was found to be the primary cause of the crash. Perhaps the greatest security challenge in railway operation is preventing the tampering of manually-operable switches: similar wrecks near Newport News, Virginia on August 12, 1992 and in Stewiacke, Nova Scotia on April 12, 2001 — both non-lethal — resulted from switches being thrown open in front of the trains by teenaged saboteurs.

Monorail systems have special switches (see the The Switch Myth, a page which includes animations).

Image:Wye switch.jpg|Often installed where space is at a premium, wye switches have trailing ends that are symmetrical and diverge in opposite directions. Image:Jointed switch.jpg|A manually-operated wye switch with joints (lower right corner shows the joint construction). Joints are not required for the electrically operated switches, as they have enough power just to bend the rail. Image:Mh gleisverschlingung.jpeg|A grooved rail turnout on a street tramway.

Components

The points are the movable rails which guide the wheels towards one diverging track or the other. They are sharpened on most switches but on a stub switch they have square ends.

The frog refers to the crossing point of two rails. This can be assembled out of several appropriately cut and bent pieces of rail or can be a single casting. The divergence of a switch is determined by the angle of the frog, with the length and placement of the other components being determined from this using established formulas and standards. This divergence is measured as the number of units of length for a single unit of separation. This is generally referred to as a switch's "number." For example, on a "number 12" switch, the rails are a distance of one foot (1'-0") apart at distance of twelve feet (12'-0") from the center of the frog. A frog also refers to a similar construction that is not part of a switch; see also "frog war."

On lines with heavy and/or high-speed traffic, a movable point frog is often used. As the name implies, there is a second set of points located at the frog. This effectively eliminates the gap in the rail that normally occurs at the frog, so long as trains are moving in the direction that the switch is aligned to. Two switch machines are required to make a movable point frog switch work.

A guard rail is a short piece of rail placed alongside the main rail opposite the frog. These exist to ensure that the wheels follow the appropriate flangeway through the frog and that the train does not derail. Generally, there are two of these for each frog, one by each outer rail. Guard rails on the outer rails are not needed with the one-piece cast frog as they are part of the casting.

A switch motor is an electric or pneumatic mechanism that aligns the points with one of the diverging routes.

A points lever, ground throw, or (in US parlance) switchstand is a lever and accompanying linkages that are used to align the points of a switch manually. This lever and its accompanying hardware is usually mounted to a pair of long sleepers that extend from the switch at the points. They are often used in a place of a switch motor on infrequently used switches.

Types of switches

Double slip

A double switch, English switch (Mainland Europe), tangle or puzzle switch or (in British English) a double slip, is a crossing of railroads "AB" and "CD" at a small angle, with the possibility of vehicles being made to pass from one to the other (go from "A" to "B" or from "C" to "D") as well as the conventional same-track passages of "A" to "D" and "C" to "B." This construction is almost equivalent to one crossing and four switches (two left and two right), or two switches back to back (track "A" to "A") but without the need to be able to set all four independently: the whole double switch has only two positions, crossing and bending.

Single slip

A single slip switch works on the same principle as a double slip but only allows the possibility of going straight through as well as going from "A" to "B" only or from "C" to "D" only. This is normally used to allow access to sidings and improve safety by avoiding having switch blades facing the usual direction of traffic.

Crossover

A crossover is a pair of switches that joins two parallel tracks.

Stub switch

A stub switch lacks the sharpened points of a typical switch. Instead the rails at the facing-point end are bent by the switch mechanism to align with the rails of one of the diverging routes, which are cut off square near where the movable points would otherwise begin. Stub switches were more common in the very early days of railways and their tramway predecessors—although some modern monorail switches use the same principle.

Stub switches are used primarily on narrow gauge lines and branch lines where the relative flexibility of the lighter rails makes this practical. Because the rails leading up to the facing-point end are not secured to the sleepers for several feet leading to the switch, and rail alignment across the gap is not positively enforced, these switches cannot be traversed at high speed and are thus not suitable for main line use. Furthermore, a points switch is much more forgiving of the error of approaching from the "wrong" or "open" trailing side.

A stub can do one thing points cannot do easily: select between three choices.^*

Three-way switches

While normally a single road would split into two roads, it is also possible to split a road into three roads, although the complexity of such arrangements usually results in severe speed restrictions. Three way switches are therefore usually only used where space in a station or depot is restricted and where low speeds are normal. For example, the Light Street tram depot in Brisbane, Queensland, had a three way set of points; this piece of trackwork is now located at the Brisbane Tramway Museum. Most three-way switches were stub switches, though some were built using points. Often these were actually two normal switches pushed so close together that they overlapped slightly. See an example here.

Derailer

A derailer can consist of a single switch point installed in a track which can be pulled away from the rail to derail any stray railroad cars which would otherwise roll onto and obstruct a main line (sometimes called "fouling") and cause a more serious accident. They are often installed on branch lines and sidings near where they connect to the main line in locations where grade or even high winds can cause an unattended car to begin rolling towards the main line.

Run-off points

Run-off points serve the same function as derailers, but are made of one or two blades of a turnout. Depending on the situation in which they are used, run-off points are referred to either as 'trap points' or 'catch points'. Trap points are provided at the exit from a siding or where a goods line joins a line that may be used by passenger trains. Catch points (now obsolescent) were installed on the running line itself, where the railway climbs at a steep gradient. These points are a throwback to the days of the 'unfitted' goods train, their purpose being to derail the rear portion of a train that had become divided, so as to prevent collision with a following train. Catch points were usually held in the 'derail' position by a spring.

Railway Switch Centralization

Is a system that controls the behavior of the Railway Switching Networks and signals at station hubs, where automation assists to operate the station properly.

High-speed turnouts

Only the plainer kinds of turnouts come in high speed versions. High speed turnouts are much longer with a finer crossing angle.

An AREMA (American Railway Engineering and Maintenance of Way Association) design number 20 turnout has a diverging speed limit of 45 miles per hour ^*. Higher speeds are possible without lengthening the turnout by using uniformly curved rail and a very low entry angle.

The conventional way to increase turnout speeds is to lengthen the turnout and use a shallower frog angle. If, however, the angle would be so shallow that a fixed frog could not support a train's wheels, a swing-nose crossing will be used.

Double slips are restricted to low-speed operation.

External links

J. B. Calvert on Turnouts and the Wharton switch in particular
MacPherson switch
ThyssenKrupp handbook

Rail technologies | Rail infrastructure | Rail junction types | Switches

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