vertically Lining cylinders Two horizontal cylinders between unit chassis and main machine chassis used to push/pull the track horizontally Clamp or "hook" assemblies These are usually on the outside of each rail and utilize hydraulic cylinders to clamp onto the side of the railhead so that is locked to the unit chassis before lifting and lining is commenced The lifting lining unit is usually secured to the main machine chassis via a hydraulically length-adjustable trailing arm. The arm is adjustable so that the unit can be moved clear of small obstructions such as insulated joints or wires. Reference system To ensure track geometry is corrected, most tamping machines use a two chord lining system (one for vertical alignment and one for horizontal alignment). The two chord system requires three reference trolleys fitted to the machine - usually called A point, B point and C point. Some machines will use a fourth point between B and C to carry out quality control measurements. A point is always the front reference point and sits on uncorrected track. The anchorage for chords on A point are able to move to compensate for defects in track geometry - this is done by the "tower" operator. Depending on the chord system, the anchorages will be either a wire anchor or a light source. B point is positioned as close as physically possible to the lifting lining unit. B point is used by the machine control system to position the track correctly using either potentiometers or optical filters, depending on type of chord system used. C point is the rear-most measuring point and anchorage point for lifting and lining chords. Depending on type of chord system C point will be either wire anchor with a tensioning cylinder or a photoelectric light receiver. All three points are individual rail trolleys able to freely move up, down, left and right independent of the machine chassis and hence follow any minor fluctuation in rail position. When working, the machine uses pneumatic cylinders to lightly push these trolleys into the selected datum rail both vertically and horizontally. When using this method, the tower operator positions A point anchorages according to existing track geometry measurements taken beforehand. Once A point has been positioned, it is then assumed that both A and C are in correct position as if machine were sitting on corrected track. The machine then uses the lifting lining