Railroad switch

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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 at the right, rail track A divides into two tracks, B (the straight track) and C (the diverging track). Each switch contains a pair of linked tapering rails (North America: points or Commonwealth: 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. This is known as a facing-point movement.

A railroad car's wheels are guided along the tracks by coning of the wheels. Only in extreme cases does it rely on the flanges located on the insides of the wheels. When the wheels reach the switch, the wheels are guided along the route determined by which of the two points is connected to the track facing the switch. In the illustration, if the left point is connected, the left wheel will be guided along the rail of that point, and the train will diverge to the right. If the right point is connected, the right wheel's flange will be guided along the rail of that point, and the train will continue along the straight track. Only one of the points may be connected to the facing track at any time; the two points are mechanically locked together to ensure that this is always the case. In the illustration at the right, the current position of the switch is for movement along the diverging track to the right.

A train coming from B or C will be led to A regardless of the position of the points, as the wheel back will force the points to the proper position. Passage in this direction through a switch is known as a trailing-point movement. If the points are rigidly connected to the switch control mechanism, the control mechanism's linkages may be bent (known as "running through the switch"), requiring repair before the switch is again usable. Therefore, switches are normally set to the proper position before performing a trailing-point movement. However, some switches, such as variable switches or spring switches, are designed to be forced to the proper position without damage.<ref>Rules 8.9, 8.15, and 8.18, General Code of Operating Rules, Fifth Edition. (c) 2005 General Code of Operating Rules Committee.</ref>. An example of mechanism, popular in the UK, that would require repair after a run-through in the trailing direction is a clamp-lock.

[edit] 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 (see Components, below). 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).

Often installed where space is at a premium, wye switches have trailing ends that are symmetrical and diverge in opposite directions.

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.

[edit] 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 and length of a switch is determined by the angle of the frog and the curvature of the switch blades, 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. In the US 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". In the UK points and crossings using chaired bullhead rail would be referred to using a letter and number combination. The letter would define the length (and hence the radius) of the switch blades and the number would define the angle of the crossing (frog). Thus an A7 turnout would be very short and likely only to be found in dockyards etc. whereas a E12 would be found as a fairly high speed turnout on a mainline.

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. In the UK, infrequently used points may be operated from a ground frame.

[edit] Types of switches

[edit] Double slip

A double slip switch (in British English simply a double slip; in North American English often simply a double switch or colloquially a puzzle switch; in Victoria a double compound) is a narrow-angled diagonal flat crossing of two lines ("AB" and "CD") combined with four pairs of points in such a way as to allow vehicles, as well as going straight across, also to pass from "A" to "D" and "C" to "B", and vice versa. Although there is thus the possibility of setting four routes; the four blades at each end of the crossing are often connected to move in unison, so the crossing can be worked by just two levers or point motors.

[edit] Single slip

A single slip switch works on the same principle as a double slip but provides for only one switching possibility (either "A" to "D" or "C" to "B") in addition to running straight across. This is normally used to allow access to sidings and improve safety by avoiding having switch blades facing the usual direction of traffic.

[edit] Outside slip

Image:2001-07-19.0004.DKW-Baeseler.jpg An outside slip switch works on the same principle as either a single or double slip but the switch blades are not contained wholly within the diamond although the rail of the road joining the outer curved rail of the two sets of switch blades passes through the diamond.

[edit] Crossover

A crossover is a pair of switches that joins two parallel tracks. Like the switches themselves, crossovers can be described as either facing or trailing.

[edit] Scissors Crossover

A scissors crossover is pair of crossovers (one facing and one trailing) overlaid such that passage may be made from the left hand road to the right hand road and vica versa. The overlaying makes for a very compact track layout at the expense of a diamond crossing in the middle.

[edit] 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 stub switch being approached from the "wrong" or "open" trailing side would very likely result in derailment.

A stub can do one thing points cannot do easily: select between three choices.[1]

[edit] Three-way switches

While normally a single road would split into two divergent paths, it is also possible to split a road three ways, 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 standard switches assembled so that they overlapped. These were extremely difficult to hold to gauge throughout the switch.

[edit] Interlaced turnout

While a three-way turnout splits a road into three divergent paths, one variant on this arrangement involves placing two standard turnouts in an "interlaced" fashion (i.e., the switch rails of the second turnout are positioned between the switch rails and the common-crossing/frog of the first turnout. In common with other forms of three way turnouts an additional common-crossing is required. Due to the inherent complexity, turnout configurations such as this are normally used only in locations where space is exceptionally tight, such as station throats or industrial areas within large cities. See an example here.

[edit] 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.

[edit] 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.

[edit] 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 [2]. 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.

[edit] Dual gauge switches

These are used with dual gauge track. In some cases, one gauge can go both directions but the other can only go one way.

[edit] Switched Diamond

Although not strictly speaking a turnout, a switched diamond is an active trackwork assembly used where the crossing angle between two tracks is too shallow for totally-passive trackwork. These vaguely resemble two standard points assembled very closely toe-to-toe. These would also often utilise swing-nose crossings at the outer ends to ensure complete wheel support in the same way as provided on very high speed i.e., shallow angle turnouts.

[edit] Video of a track moving

[edit] References

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[edit] See also

• Rail terminology (US/UK differences highlighted)
• Expedition Everest, first roller coaster with switches.
• double junction
• Railway switching networks
• Centralized Traffic Control
• Railway signalling

[edit] External links

• J. B. Calvert on Turnouts and the Wharton switch in particular
• MacPherson switch
• ThyssenKrupp handbookbg:Стрелка

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