Railroad switch
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A railroad switch (known in British and Australian English as (a set of) points or, in technical usage, a turnout) is a mechanical installation provided at a point where rail track A divides into two tracks B and C. It can be set in either of two positions, determining whether a train coming from A will be led to B or to 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 in thus "forcing" a switch at all but the slowest speeds 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 too. End A is referred to as the facing-point end.
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(picture shows a model railway switch).
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The switch contains a pair of linked tapering rails that can be moved laterally (point blades).
The position of the switch is usually changed electrically on main lines and controlled from a remote control center or 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 tower 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 symmetric, 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. Thankfully the movement of the switch occurred beneath the final coach, so that although seven people were killed, the front coaches were spared. Poor maintenance of the points was held to be the primary cause of the crash. (Perhaps the greatest security challenge in railway operation is preventing the tampering of manually-operable switches: a similar wreck in Stewiacke, Nova Scotia, April 12, 2001 — luckily nonlethal — was caused by a switch being unlocked by "bored" teenagers.)
Monorail systems have special switches, see the external link (http://www.monorails.org/tMspages/switch.html), a page which includes animations.
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.
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The frog (or in British usage, nose) 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 switches 'number.' For example, on a 'number 12' switch, the rails are a distance of one foot apart a distance of twelve feet 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.
A switch motor is an electric or pneumatic mechanism that aligns the points with one of the diverging routes.
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A points lever, ground throw, or switchstand (US parlance) 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 seldomly used switches.
Types of Switches
Double Slip
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A double switch or English switch, known in British English as 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 D and from B to C). 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 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.
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: select between three choices.[1] (http://www.spikesys.com/EBT/Tour/3way.html)
Three way switches
If a station is short of space, it is possible up to a point to build switches on top of one another. This increases the number of frogs, and if the switch-blades themselves overlap, probably imposes speed restrictions.
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 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.
Catchpoints
Catchpoints serve the same function as derailers, but are made of one or two blades of a turnout.
High Speed Switches
Only the plainer kinds of turnouts come in high speed versions. High speed turnouts are much longer with a finer crossing angle.
Double slips are restricted to low speed operation.
Gantlet track
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A gantlet track (in British and Australian English: gauntlet, or gauntleted, track) refers to the situation where tracks converge onto a single roadbed and are interlaced to pass through a narrow passage such as a cut, bridge, or tunnel. A switch frog at each end allows the two tracks to overlap, and the four rails run parallel through the passage on the same crossties (sleepers) and separate again at the other end. Gantlet tracks are commonly used when a rail line's capacity is increased with the addition of an additional track, but cost or other factors prevent the widening of the bridges. Since there are no points or other moving parts in a gantlet track, a train operating on one of the tracks cannot be routed onto the other. Because two trains cannot use the gantlet at the same time, scheduling and block signals must allow for this restriction.
An alternative arrangement is to use three rails (dual gauge), with the two tracks sharing the middle rail. Gantlet track is typically used for short stretches of track where it is cheaper to provide extra rails than to provide switches and reduce the line to single track.
A gantlet track can also be used when two railroads of different gauges share right-of-way; the standard-gauge Delaware, Lackawanna and Western Railroad used the wide-gauge Erie Railroad's tunnel through the New Jersey Palisades in this way before the DL&W built its own tunnel.
The term is derived not from gauntlet meaning a type of glove, but from the expression running the gauntlet, which means running between two confining rows of adversaries.
See also
- Rail terminology (US/UK differences highlighted)
- Expedition Everest, first roller coaster with switches
- A section of gauntlet track on Croydon Tramlink
External links
- MacPherson switch (http://www.catskillarchive.com/rrextra/tkmacph.Html)
- J. B. Calvert (http://www.du.edu/~jcalvert/) on Turnouts (http://www.du.edu/~jcalvert/railway/turnout.htm) and the Wharton switch (http://www.du.edu/~jcalvert/railway/wharton.htm) in particular
- Gauntlet track (http://www.midtowngreenway.org/programs/transit/Trolley_Study_HTML/4routealignment.htm) - four rails in Minneapolis (with diagram)
- Gauntlet track (http://www.streetcar.org/yesterday/rejuvenation/) - three rails in San franciscode:Eisenbahnweiche