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A point machine is used to allow railway trains to switch tracks smoothly, which is usually called a railroad turnout. The initial purpose of the point machine was to be able to control the track and direction of the train from a distance. Some current versions of the point machine have installed self-lubricating and timing features. There are several types of this mechanism that operate on different power sources but serve similar purposes. The type of device used generally depends on the railroad technician, the type and weight of trains most frequently using the tracks, and number of employees available to perform maintenance.
The key principles in choosing a point machine are reliability, low-maintenance, economic feasibility, and universal suitability. Railroads often have numerous devices, and each electric point machine is usually connected to a mainframe computer which is connected to a number of personal computers. The use of computers helps operators view the status or possible malfunction of the machine. Computers in the control center usually record all necessary data to asses the functionality of each point machine, and employees can determine whether repair or modification is needed.
Rails guide the wheels of trains and when the point machine switches the point where rails meet to a different position, the train changes course. For instance, a train that approaches a fork is given the option to go straight or veer right, and the decision is made by the control operator who uses the point machine to perform the switch. This was once controlled manually, and sometimes involved a mechanic hopping out of the train to pull a lever in order to switch tracks. Further innovations made it possible to control point machines from short distances, and more recently there have been system upgrades to increase the distance of the control centers. Public transportation systems are an excellent example, because most systems have a main control room that runs a city's tracks.
The appearance of point machines vary, but the most common looks like a small metal box usually made from cast iron with vents and a lock. A motor and other mechanical components are protected by the cast iron box, and the ventilation is used to release heat produced by the motor. There is a transmission clutch that is used to control the throwing force and detector slides to determine the position of the switch blades, as well as the completion of the track switch. These machines are usually equipped with hand cranks for manual switching in case there is a system failure.
What is the basic/common criteria in evaluating the point machine?