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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. OPERATION , 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 Flange s 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. Rules 8.9, 8.15, and 8.18, ''General Code of Operating Rules, Fifth Edition''. (c) 2005 General Code of Operating Rules Committee.. 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. 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 (see below for what a frog is). If the frog angle is so shallow that a fixed frog cannot support a train's wheels, a Swingnose Crossing 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. Increasing speed through turnouts , from FindArticles. A higher speed turnout , from Federal Railroad Administration, US Department of Transportation. Operation in cold conditions 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. 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. Tram and monorail systems , Ireland, in 1912]] The switch points of Tram lines are often operated remotely by the Driver . Monorail systems have Special Switches . 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 a E12 would be found as a fairly high speed turnout on a mainline. 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 Saboteur s. 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 Dublin - Cork 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. 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 Lever s. The levers were also used to control Railway Signal s to control the movement of trains over the points. Eventually, mechanical systems known as Interlocking s 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 Yard s, or on Heritage Railway s, switches may still be operated in this way. COMPONENTS 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. 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''). 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. 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 . 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. 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. 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. Components gallery |
Image:Bewegliches HerzstueckjpgA ''
| "http://wwwinformationdelightinfo/information/entry/swingnose_crossing" class="copylinks">Swingnose Crossing '' The point of the V-shaped rail is moved to align the rail in the appropriate direction where the two rails cross |
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Image:FAW-points1jpgThe ''switch Motor'' (in This Case An
| "http://wwwinformationdelightinfo/information/entry/electric_motor" class="copylinks">Electric Motor ) and associated mechanism used to operate this switch can be seen to the right of the picture |
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Image:GroundframejpgA ''
| "http://wwwinformationdelightinfo/information/entry/ground_frame" class="copylinks">Ground Frame '' (at Bristol Temple Meads station, UK) contains a few levers for manually operating nearby points |
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