There is a ring of protruding rims on the inner side of the train wheel, which is just stuck on the inner side of two rails. The function of turnout is to change the driving direction by acting on the rim. The front end of each turnout is two movable guide rails, which are controlled by the switch machine and can be attached to the front or lateral tracks to guide the wheels. And the end of each switch has an X-shaped throat. There is a certain gap in the middle, which allows wheels in all directions to pass. At the same time, this gap is called harmful space, which will accelerate the wear of wheel tread. On the other side of the throat, there is a wheel guard guide rail to ensure that the rim can pass through the throat smoothly and will not get stuck in the wrong direction. In addition, if the train is driving in the opposite direction of the fork, and the switch does not turn to the direction that the train should pass, an accident will occur, which is called crowding the switch. \x0d\ Turnout is a line connecting device to transfer locomotives and vehicles from one track to another, which is usually laid in large quantities in stations and marshalling stations. With turnouts, the capacity of the line can be fully exerted. Even for a single-track railway, it is possible to split the train by laying turnouts and building turnout lines longer than the length of the train. \x0d\\x0d\ Turnouts, together with curves and joints, are called the three weak links of track because of their large number, complex structure, short service life, limited train speed, low driving safety and large maintenance investment. There are three basic forms: connection, crossing and combination of connection and crossing. Common lines connect various types of single turnouts and multiple turnouts; There are straight-line intersections and diamond-shaped intersections; Connecting combination includes crossing switch and crossing line. \ x0d \ x0d \ turnout is a big family, and the most common one is the ordinary single turnout. It consists of switch, connector, frog and guardrail. Turnouts include basic rails, switch rails and switch machinery. When the locomotive and rolling stock want to transfer from lane A to lane B, operate the switch machine to move the switch rail, and the switch rail 1 clings to the basic rail 1, and the switch rail 2 is separated from the basic rail 2, so that the lane B is opened and the lane A is closed, and the locomotive and rolling stock enters the connecting part and transits to the frog and the rail guard unit along the guide curve rail. The unit includes a fixed frog center, wing rails and guard rails to protect the wheels from crossing the two rails. \ x0d \ x0d \ You may have a gap between the narrowest part of the two wing rails and the sharpest part of the center of the frog when crossing the frog, which is the harmful space of the turnout. When the wheel passes here, it may derail due to the wrong frog groove. This is the purpose of setting the guardrail, which is to forcibly guide the running direction of the wheels. Nevertheless, the existence of this harmful space limits the speed of trains passing through turnouts, which is very unfavorable for running high-speed trains. \ x0d \ x0d \ The fundamental way to solve the harmful space of turnout is of course to eliminate the harmful space. Since ordinary turnout can't do it, it is necessary to develop a special turnout-movable central track turnout. \ x0d \ x0d \ The main feature of the movable rail is that the frog rail can move. When we want to open a rail in a certain direction, the frog rail of the movable rail is close to the same wing rail and separated from the other wing rail, so that the harmful space of the ordinary turnout does not exist. Practice has proved that the harmful space of the turnout is eliminated, the traffic is more stable, and the speed limit of the turnout at the crossing is less, which is especially suitable for the lines with large traffic volume and high train speed. \ x0d \ x0d \ Since there are single turnout, there are double turnout, triple turnout and multi-turnout (multi-turnout). \ x0d \ x0d \ Double turnout is Y-shaped, that is, the two lanes connected with the turnout branch extend to both sides. \ x0d \ x0d \ A three-way switch is like a ψ shape, which connects three lanes at the same time. Two sets of switch machines operate two sets of switch rails. \ x0d \ x0d \ composite turnout is X-shaped, which is actually composed of four groups of one-way turnouts and a pair of diamond crossings. \x0d\\x0d\ In addition, there is a frequency divider, which is usually called diamond frequency divider. It consists of two groups of acute frog and two groups of obtuse frog, but there is no turnout, so it is impossible to change lanes between tracks. \x0d\\x0d\ If the upper two points and the lower two points of the X-shape of the composite turnout are connected respectively, it is a crossing line. It can not only drive in more directions, but also occupy less land, so it is often used in stations. \ x0d \ x0d \ switches have their own codes, such as No.9 switch, 12 switch, 18 switch and so on. This code is not randomly arranged, but actually represents the cotangent value of the frog angle (α), that is, the ratio of two right-angled sides FE and AE of the right-angled triangle at the center of the frog, that is, N=ctgα=FE/AE, and n is the turnout number. Obviously, the smaller the frog angle α is, the larger the value of n is, the larger the radius of the guide curve is, the more stable the train side line passes through the turnout, and the higher the allowable turnout speed is. Therefore, the use of large turnouts is beneficial to train operation. However, everything has two sides. The more turnouts, the longer turnouts, the higher the cost and the more land occupied. Therefore, the number of turnouts to be used should be adapted to local conditions and vary from line to line, and it cannot be generalized.