Swingnose crossing
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A swingnose crossing (moveable point frog in US usage) is a device used at a railway turnout to eliminate the gap at the common crossing (a.k.a. frog).
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[edit] Fixed crossing
The fixed crossing can only be so fine an angle, say 1 in 20, before the wheels start to go the wrong way at the V of the V-crossing. This limits the maximum speed of the crossing.
In addition, the gap at a fixed V-crossings are a weak point on the railway line where the heavy wheel must bump across the gap of about 10cm, supported by the wheel flanges, pounding the rail so much that the steel can deform and/or wear away. The damage spreads to other components including the wheels, and the noise can also be a nuisance. Repairing the damage can also be a problem if the railways are busy with trains, and few opportunities exist between trains to do the repairs.
[edit] Swingnose crossing
Swingnose crossings allow the V-crossing to have as fine an angle as required, allowing for higher speeds.
The swingnose crossing also overcomes the weakness of the gap by moving the point of the V-crossing from side to side depending on which way the turnout or switch diamond is set. The wheels are supported the whole way on their treads and do not make the noise and vibration that would otherwise occur.
[edit] Parameters
Swingnose crossings would be used for V-crossings finer than about 1 in 20, and for axle loads in excess of 25T.
[edit] Interlocking
Swingnose crossings need to operate at the same time as the turnout switches to which they belong. This may be too stiff for manual operation, and may require power operation.
[edit] Switched diamonds
Swingnose crossings eliminate the gap in the V-crossings (aka frog).
Switched diamonds eliminate the gap in the K-crossings.
