Railroad switch


A railroad switch, turnout, or points is a mechanical installation enabling railway trains to be guided from one track to another, such as at a railway junction or where a spur or siding branches off.

Design

The parts of a turnout are known by different names in different jurisdictions. The main terms in US and UK usage are shown in the selectable diagrams. In this article, the US term is listed first and UK second, in parentheses.
The most common type of switch consists of a pair of linked tapering rails, known as points, lying between the diverging outer rails. These points can be moved laterally into one of two positions to direct a train coming from the point blades toward the straight path or the diverging path. A train moving from the narrow end toward the point blades is said to be executing a facing-point movement.
For many types of switch, a train coming from either of the converging directions will pass through the switch 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 and switches that allow this type of movement without damage to the mechanism are called trailable switches. In turn, these can either be non-resettable or resettable, meaning they either require on-site intervention to return the switch to operation after a trailing point movement or can continue operating with no in-person actions.
A switch generally has a straight "through" track, such as the mainline, and a diverging route – respectively termed "normal" and "reverse" routes. The handedness of the installation is described by the side that the diverging track leaves. Right-hand switches have a diverging path to the right of the straight track, when coming from the point blades, and a left-handed switch has the diverging track leaving to the opposite side. In many cases, such as rail yards, many switches can be found in a short section of track, sometimes with switches going both to the right and left. Sometimes a switch merely divides one track into two; at others, it serves as a connection between two or more parallel tracks, allowing a train to switch between them. In many cases, where a switch is supplied to leave a track, a second is supplied to allow the train to reenter the track some distance down the line; this allows the track to serve as a siding, allowing a train to get off the track to allow traffic to pass, and also allows trains coming from either direction to switch between lines; otherwise, the only way for a train coming from the opposite direction to use a switch would be to stop, and reverse through the switch onto the other line, and then continue forwards.
A straight track is not always present; for example, both tracks may curve, one to the left and one to the right, or both tracks may curve, with differing radii, while still in the same direction. Switches consume a relatively high proportion of a railway maintenance budget.

History

Simple single-bladed switches were used on early wooden railways to move wagons between tracks. As iron-railed plateways became more common in the eighteenth century, cast iron components were made to build switches with check rails. In 1797, John Curr described the system that he developed which used a single iron blade, hinged on a vertical pin that was tapered to lie against the plateway. By 1808, Curr's basic design was in common use.
The use of a sprung rail, giving a smooth transition, was patented by Charles Fox in 1838.
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. Purely mechanical interlockings were eventually developed into integrated systems with electric control. On some low-traffic branch lines, in self-contained marshalling yards, or on heritage railways, switches may still have the earlier type of interlocking.

Operation

A railroad car's wheels normally take up a position over the center of the rails by virtue of the wheel treads' coning; the flanges on the inside edges 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 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. Historically, this would require a lever to be moved by a human operator, and some switches are still controlled this way. However, most are now operated by a remotely controlled actuator called a switch machine or point machine, which may contain an electric motor or a pneumatic or hydraulic actuator. This both allows remote control and monitoring, and the use of stiffer, strong switches that would be too difficult to move by hand.
In a trailing-point movement, the flanges on the wheels will force the points to the proper position. This is sometimes known as running through the switch. Some switches are designed to be forced to the proper position without damage. Examples include variable switches, spring switches, and weighted switches.
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.

High-speed operation

Generally, switches are designed to be safely traversed at low speed. However, it is possible to modify the simpler 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. On European high-speed lines, it is not uncommon to find switches where a speed of or more is allowed on the diverging branch. Switches were passed over at a speed of during the French world speed run of April 2007.
The US Federal Railroad Administration has published the speed limits for higher-speed turnouts with No. 26.5 turnout that has speed limit of and No. 32.7 with speed limit of.

Operation in cold conditions

Under cold weather conditions, snow and ice can prevent the proper movement of switch or [|frog] point rails, essentially inhibiting the proper operation of railroad switches. Historically, railway companies have employees keep their railroad switches clear of snow and ice by sweeping the snow away using switch brooms, or gas torches for melting ice and snow. Such operation are still used in some countries, especially for branch routes with only limited traffic. Modern switches for heavily trafficked lines are typically equipped with switch heaters installed in the vicinity of their point rails so that the point rails will not be frozen onto the stock rail and can no longer move. These heaters may take the form of electric heating elements or gas burners mounted on the rail, a lineside burner blowing hot air through ducts, or other innovative methods to keep the point & stock rails above freezing temperatures. Where gas or electric heaters cannot be used due to logistic or economic constraints, anti-icing chemicals can sometimes be applied to create a barrier between the metal surfaces to prevent ice from forming between them. Such approaches however, may not always be effective for extreme climates since these chemicals will be washed away over time, especially for heavily thrown switches that experience hundreds of throws daily.
Heating alone may not always be enough to keep switches functioning under snowy conditions. Wet snow conditions, which generate particularly sticky snow and whiteout conditions, may occur at temperatures just below freezing, causing chunks of ice to accumulate on trains. When trains traverse over some switches, the shock, vibration, possibly in combination with slight heating caused by braking or a city microclimate, may cause the chunks of ice to fall off, jamming the switches. The heaters need time to melt the ice, so if service frequency is extremely high, there may not be enough time for the ice to melt before the next train arrives, which will then result in service disruptions. Possible solutions include installing higher capacity heaters, reducing the frequency of trains, or applying anti-icing chemicals such as ethylene glycol to the trains.

Classification

The divergence and length of a switch is determined by the angle of the frog and the angle or 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 of the switch blades and the number would define the angle of the [|crossing]. Thus an A7 turnout would be very short and likely only to be found in tight places like dockyards whereas an E12 would be found as a fairly high speed turnout on a mainline.
On the London, Midland and Scottish Railway, switch curvatures were specified from A to F, with the following corresponding radii:
  • B – – simple [|crossover] with a 1 in 8 crossing angle
  • C – – scissors or simple crossover with a 1 in 10 crossing angle
  • D – – double track junction switch with a 1 in 12 crossing angle