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Juni 18, 2008 at 2:58 am Tinggalkan komentar

Railroad switch

A right-hand railroad switch.

A right-hand railroad switch.

Animated diagram of a right-hand railroad switch showing principle of operation.

Animated diagram of a right-hand railroad switch showing principle of operation.

This article primarily uses North American terminology. British and Commonwealth terms are given in brackets.

A railroad switch is a mechanical installation enabling trains to be guided from one track to another. In the UK and Commonwealth countries, railroad switches are known as (sets of) points. In technical usage switches are also called turnouts.

In the diagram on the right, rail track A divides into two: track B (the straight track) and track C (the diverging track). The switch consists of the pair of linked tapering rails, known as points (switch rails or point blades), lying between the diverging outer rails (the stock rails). These points can be moved laterally into one of two positions so as to determine whether a train coming from A will be led towards B or towards C. A train moving from the A direction towards either B or C is said to be executing a facing-point movement.

Unless the switch is locked, a train coming from B or C will be led to A regardless of the position of the points, as the vehicle’s wheels will force the points to move. Passage through a switch in this direction is known as a trailing-point movement.

A switch can be described by the direction in which the diverging track leaves the straight track. A right-hand switch has track C to the right of a straight track formed by A and B. A left-hand switch has track C to the left.

A straight track is not always present; for example, both tracks may curve, one to the left and one to the right (see Wye switch, below) or both tracks may curve, with differing radii, in the same direction.



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[edit] Operation

The operation of a railroad switch. In this animation, the red track is the one travelled during a facing-point movement. The switch mechanism, shown in black, may be operated remotely using an electric motor or lever or from a nearby ground frame.

The operation of a railroad switch. In this animation, the red track is the one travelled during a facing-point movement. The switch mechanism, shown in black, may be operated remotely using an electric motor or lever or from a nearby ground frame.

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.

A mechanism is provided to move the points from one position to the other (change the points). Historically, this would require a lever to be moved by a human operator, and some switches are still controlled in this way. However, most are now operated by a remotely controlled electric motor or by pneumatic or hydraulic actuation.

In a trailing-point movement, the wheels will force the points to the proper position. This is sometimes known as running through the switch. If the points are rigidly connected to the switch control mechanism, the control mechanism’s linkages may be bent, requiring repair before the switch is again usable. For this reason, switches are normally set to the proper position before performing a trailing-point movement. [1].

An example of mechanism that would require repair after a run-through in the trailing direction is a clamp-lock. This mechanism is popular in the UK, but the damage caused is common to most types of switches. However, some switches are designed to be forced to the proper position without damage. Examples include variable switches, spring switches, and weighted switches.

[edit] High-speed operation

Generally, switches are designed to be safely traversed at low speed. However, it is possible to modify the more simple types of switch to allow trains to pass at high speed. More complicated switch systems, such as double slips are restricted to low-speed operation.

The conventional way to increase turnout speeds is to lengthen the turnout and use a shallower frog angle. If the frog angle is so shallow that a fixed frog cannot support a train’s wheels, a swingnose crossing (US: moveable point frog) will be used. Higher speeds are possible without lengthening the turnout by using uniformly curved rail and a very low entry 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][3]

[edit] Operation in cold conditions

Gas heating keeps a switch free from snow and ice.

Gas heating keeps a switch free from snow and ice.

In cold conditions, snow and ice can prevent the correct operation of switches. In the past, people were employed by railway companies to keep the switches clear by sweeping the snow away, and this is still used in some countries, especially on minor lines. Some were provided with gas torches for melting ice. More recently, switches have had heaters installed in the vicinity of the points so that the temperature of the rails in these areas can be kept above freezing. The heaters may be powered by gas or electricity.

[edit] Tram and monorail systems

A switch on the Listowel and Ballybunion Railway, Ireland, in 1912

A switch on the Listowel and Ballybunion Railway, Ireland, in 1912

The switch points of tram lines are often operated remotely by the driver. Monorail systems have special switches.

[edit] Roller coaster switches

Many roller coasters have switches for the siding, or even for a double station system, for example in Disneyland Resort ParisSpace Mountain and Big Thunder Mountain coasters.

Regular rail can cross its own track because the gaps in the rails for wheel flanges are narrow, permitting the bladed design in this article. Round pipe roller coaster rails and box beam monorail rails usually have wheels riding at angles other than on top. These additional other angle wheels are a larger loading gauge, requiring big gaps in the rail (structure gauge) where rails cross or meet.

There are three basic switch designs for roller coasters. Flexing, substituting and table rotating rails have all been used. Flexing the entire rail truss, fixed at one end, to point towards an alternate destination requires manipulating a long segment of rail. Substituting a segment requires placing two or more segments of rail on flat plate that is moved in its entirety to provide straight or curved track.

Substitution track switch for rail at Chester Zoo

Substitution track switch for rail at Chester Zoo

Alternately these substitution track segments can be wrapped around a rotating cylinder, creating a triangular truss or a two sided plate. Rotating a table with a curved track segment in a Y junction is the less used third option. This can even make a triangle junction. If the curved track turns the cars 60 degrees, and three rail lines meet as three equally spaced spokes, 120 degrees apart, than the curved track sitting on a turn table can be rotated to connect any two of the three rail lines at this junction.

[edit] Classification

The divergence and length of a switch is determined by the angle of the frog (the point in the switch where two rails cross, see below) and the curvature of the switch blades. The length and placement of the other components are 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 North America this is generally referred to as a switch’s “number”. For example, on a “number 12” switch, the rails are one unit apart at a distance of twelve units from the center of the frog. In the United Kingdom 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 an E12 would be found as a fairly high speed turnout on a mainline.

[edit] Safety

The correct setting of points is fundamental to the safe running of a railway. For example, an incorrectly set switch may result in two trains being on the same track, causing a collision.

Perhaps the greatest security challenge in railway operation is preventing the tampering of manually-operable switches. Similar (non-fatal) wrecks near Newport News, Virginia on August 12, 1992 and in Stewiacke, Nova Scotia on April 12, 2001 resulted from switches being thrown open in front of the trains by teenaged saboteurs. To prevent these incidents, most unused switches are locked up.

In 1980, 18 people died in the Buttevant Rail Disaster at Buttevant, Co. Cork in Ireland, when the DublinCork express was derailed at high speed after being inadvertently switched into a siding via ground frame operated points.

The Potters Bar rail crash at Potters Bar, Hertfordshire in the United Kingdom occurred in May 2002, when a switch sprang to a different position as a coach crossed it, a type of mishap called “splitting the switch.” The front wheels of a coach progressed along the straight track as intended, but the rear wheels slewed along the diverging track. This caused 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. The initial conclusion of the enquiry into the Grayrigg derailment of 23 February 2007 blames an incorrectly maintained set of points.

[edit] History

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.

Prior to the widespread availability of electricity, switches at heavily-traveled junctions were operated from a signal box constructed near the tracks through an elaborate system of rods and levers. The levers were also used to control railway signals to control the movement of trains over the points. Eventually, mechanical systems known as interlockings were introduced to make sure that a signal could only be set to allow a train to proceed over points when it was safe to do so. On some low-traffic branch lines, in self-contained marshalling yards, or on heritage railways, switches may still be operated in this way.

[edit] Components

[edit] Points (point blades)

The points (switch rails or point blades) are the movable rails which guide the wheels towards either the straight or the diverging track. They are tapered on most switches, but on stub switches they have square ends.

In the UK and Commonwealth countries, the term points refers to the entire mechanism, whereas in North America the term refers only to the movable rails.

[edit] Frog (common crossing)

The frog (common crossing) 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. A frog forms part of a railroad switch, and is also used in a level junction (flat crossing). The frog is a point of weakness because the wheels are unsupported for a short distance and can inflict wear and damage. There is also a small risk that the wheels may go the wrong way.[citation needed]

On lines with heavy and/or high-speed traffic, a swingnose crossing 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.

This use of the word “frog” derives from the appearance of the triangular assemblage of rails which recalls the frog of a horse’s hoof.

[edit] Guard rail (check rail)

A guard rail (check rail) is a short piece of rail placed alongside the main (stock) 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 are not required with a “self-guarding cast manganese” frog, as the raised part of the casting serve the same purpose. These frogs are for low-speed use and are common in rail yards.

[edit] Switch motor

A switch motor (also known as a switch machine or point machine) is an electric or hydraulic or pneumatic mechanism that aligns the points with one of the possible routes. The switch motor also includes electrical contacts to detect that the switch has completely set and locked. If the switch fails to do this, signals are kept at red.

[edit] Points lever

A points lever, ground throw, or 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 some places, infrequently used points may be operated from a ground frame. To prevent the tampering with of these switches by outside means, these switches are locked up when not in use.

[edit] Joints

Joints are used where the moving points meet the fixed rails of the switch. They allow the points to hinge easily between their positions. Originally the movable switch blades were connected to the fixed closure rails with loose joints, but since steel rails are somewhat flexible it is possible to make this join by thinning a short section of the rail itself. This can be called a heelless switch.

[edit] Components gallery

[edit] Types of switches

A double switch, or double slip.

A double switch, or double slip.

Apart from the standard right-hand and left-hand switches, switches commonly come in various combinations of configurations.

[edit] Double slip

A double slip switch (double slip) is a narrow-angled diagonal flat crossing of two lines combined with four pairs of points in such a way as to allow vehicles to change from one straight track to the other, as well as going straight across. A train approaching the arrangement may leave by either of the two tracks on the opposite side of the crossing. To reach the third possible exit, the train must change tracks on the slip and then reverse.

The arrangement gives the possibility of setting four routes, but 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.

In North America, the arrangement may also be called a double switch, or more colloquially, a puzzle switch. The Great Western Railway in the United Kingdom used the term double compound points, and the switch is also known as a double compound in Victoria (Australia).

[edit] Single slip

Outside slip in Heidelberg main station

Outside slip in Heidelberg main station

A single slip switch works on the same principle as a double slip but provides for only one switching possibility. Trains approaching on one of the two crossing tracks can either continue over the crossing, or switch tracks to the other line. However, trains from the other track can only continue over the crossing, and cannot switch tracks. 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

An outside slip switch is similar to both the double and single slips, but the switch blades are not contained wholly within the diamond. A semi-outside slip is one where the switch blades are partly contained within the diamond. A fully-outside switch is one where the switch blades do not enter the diamond.

[edit] Crossover

two pairs of switches linking two tracks to each other in both directions.

A scissors crossover: two pairs of switches linking two tracks to each other in both directions.

A crossover is a pair of switches that connects two parallel rail tracks, allowing a train on one track to cross over to the other. Like the switches themselves, crossovers can be described as either facing or trailing.

When two crossovers are present in opposite directions, one after the other, the four-switch configuration is called a double crossover. If the crossovers overlap it is dubbed a scissors crossover, scissors crossing, or just scissors; or, due to the diamond in the center, a diamond crossover. This makes for a very compact track layout at the expense of using a level junction.

In a setup where each of the two tracks normally carries trains of only one direction, a crossover can be used either to detour “wrong-rail” around an obstruction or to reverse direction. A crossover can also join two tracks of the same direction, possibly a pair of local and express tracks, and allow trains to switch from one to the other.

On a crowded system, routine use of crossovers (or switches in general) will reduce throughput, as the switches must be changed for each train. For this reason, on some high-capacity rapid transit systems, crossovers between local and express tracks are not used during normal rush hour service, and service patterns are planned around use of the usually flying junctions at each end of the local-express line.

[edit] Stub switch

A narrow gauge stub switch.

A narrow gauge stub switch.

A stub switch lacks the tapered points (point blades) of a typical switch. Instead, both the movable rails and the ends of the rails of the diverging routes have their ends cut off square. The switch mechanism aligns the movable rails with the rails of one of the diverging routes.

The rails leading up to a stub switch are not secured to the sleepers for several feet, and rail alignment across the gap is not positively enforced. Stub switches also require some flexibility in the rails, or an extra joint at which they hinge. Therefore these switches cannot be traversed at high speed or by heavy traffic and so are not suitable for main line use. A further disadvantage is that a stub switch being approached from the diverging route that is not connected by the points would result in a derailment.

Stub switches were more common in the very early days of railways and their tramway predecessors. Now, because of their disadvantages, stub switches are used primarily on narrow gauge lines and branch lines. Some modern monorail switches use the same principle.

[edit] Plate switch

A narrow gauge plate switch

A narrow gauge plate switch

A plate switch incorporates the tapered points of a typical switch into a self-contained plate. Each point blade is moved separately by hand. Plate switches are only used for double-flanged wheels, with wheels running through the plates on their flanges, guided by the edges of the plate and the moveable blade.

Because plate switches can only be used by double-flanged wheels and at extremely low speeds, they are typically only found on hand-worked narrow gauge lines.

[edit] Three-way switch

A three-way stub switch at Sheepscot station on the Wiscasset, Waterville and Farmington Railway

A three-way stub switch at Sheepscot station on the Wiscasset, Waterville and Farmington Railway

A three-way switch is used to split a railroad track into three divergent paths rather than the more usual two. The complexity of such arrangements usually results in severe speed restrictions, and therefore three-way switches are usually only used in a station or depot where space is restricted and low speeds are normal.

Stub switches can more readily select between three routes, so most three-way switches are stub switches, although some were built using points.[1] It was extremely difficult to hold the two rails the correct distance apart for the length of the switch with these types of switch.

A three-way switch formerly at Brisbane's Light Street tram depot now on display at the Brisbane Tramway Museum.

A three-way switch formerly at Brisbane‘s Light Street tram depot now on display at the Brisbane Tramway Museum.

A three-way switch from a Brisbane tram depot is shown on the right. This example has two points (point blades) on each track, allowing for three diverging routes. The points can both be set to one side, resulting in a vehicle turning off the straight track. Alternatively, the two blades can be separated if the vehicle must continue along the straight track.

[edit] Interlaced turnout

A pair of interlaced turnouts on the Loop in Chicago.

A pair of interlaced turnouts on the Loop in Chicago.

An interlaced turnout is a different method of splitting a track into three divergent paths. It is an arrangement of two standard turnouts, one left- and one right-handed, in an “interlaced” fashion. The points of the second turnout are positioned between the points and the 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 of the arrangement, interlaced turnouts are normally only used in locations where space is exceptionally tight, such as station throats or industrial areas within large cities. Interlaced turnouts can also be found in some yards, where a series of switches branching off to the same side are placed so close together that the points of one switch are placed before the frog of the preceding switch. (example)

[edit] Wye switch

A wye switch.

A wye switch.

A wye switch (Y points) has trailing ends which diverge symmetrically and in opposite directions. The name originates from the similarity of their shape to that of the letter Y. Wye switches are usually used where space is at a premium. In North America this is also called an “Equilateral Switch” or “Equilateral Turnout”

[edit] Run-off points

Main article: Catch points
Trap points at the exit from a yard

Trap points at the exit from a yard

Run-off points are used to protect main lines from stray or runaway railroad cars or from trains passing signals set at danger. In these cases, vehicles would otherwise roll onto and obstruct a main line (sometimes known as fouling) and cause an accident. Depending on the situation in which they are used, run-off points are referred to either as trap points or catch points. Derailers are another device used for the same purpose.

Catch points are installed on the running line itself, where the railway climbs at a steep gradient. They are used to prevent runaway vehicles colliding with a train or other equipment (such as level crossing barriers) further down the slope. In some cases, catch points lead into a sand drag to safely stop the runaway vehicle, which may be travelling at some speed. Catch points are usually held in the ‘derail’ position by a spring. They can be set to allow a train to pass safely in the downhill direction using a lever or other mechanism to override the spring for a short time.

Catch points originate from the days of the ‘unfitted’ goods train. These trains did not have a mechanism in place to automatically brake runaway cars. Catch points were therefore required to stop the rear portion of a train that had become divided, although they would also stop vehicles that had run away for any other reason. Now that trains are all ‘fitted’, catch points are mostly obsolete.

Similar to 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. Unless they have been specifically set to allow traffic to pass onto the main line, the trap points will direct any approaching vehicle away from the main line. This may simply result in the vehicle being derailed, but in some cases a sand drag is used, especially where the vehicle is likely to be a runaway travelling at speed due to a slope.

[edit] Derailers

Main article: Derail (railroad)

A derailer works by derailing any vehicle passing over it. There are different types of derailer, but in some cases they consist of a single switch point installed in a track. The point can be pulled into a position to derail any equipment that is not supposed to pass.

[edit] Dual gauge switches

A dual gauge switch in Japan.

A dual gauge switch in Japan.

Dual gauge switches are used in dual gauge systems. There are various possible scenarios involving the routes that trains on each gauge may take, including the two gauges separating or one gauge being able to choose between diverging paths and the other not. Because of the extra track involved, dual gauge switches have more points and frogs than their single gauge counterparts. This limits speeds even more than usual.

[edit] Switch diamond

A switch diamond at a junction in the UK

A switch diamond at a junction in the UK

Although not strictly speaking a turnout, a switch 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 swingnose crossings at the outer ends to ensure complete wheel support in the same way as provided

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