sections which have no points or other complex track structures, commonly referred to as production machines, generally have fixed tamping head positions Duomatic Continuous action tamping machines Tamping Express Switch tamping machines: used to tamp switches, crossings and other complex track structures, have movable tamping heads with ability to isolate heads when required. Tamping process The tamping process from any type of tamper consists of the following basic steps: The lifting, lining unit moves the rail and sleepers under the tamping unit into the desired vertical and horizontal position. At this stage, the desired position of the rails and sleeper will only be maintained as long as the lifting, lining unit holds the track in position. The tamping units drive vibrating and oscillating tines (35 Hz optimum) into the ballast on both sides of the sleeper parallel to the rails until squeezing depth is achieved. The rapidly vibrating and oscillating tines in effect liquefy the ballast to allow easy insertion of the tines. The vibrating tines are pushed together underneath the sleeper to pack ballast underneath the sleeper being held in position by the lifting, lining unit to ensure it holds position when released by the lifting, lining unit. The tamping unit retracts to the rest position, slightly above the top of the rails. The track and sleeper are released by the lifting, lining unit and maintain their position thanks to the modification of the ballast done in the previous step of the process. The tamping machine moves to the next sleeper and begins a new cycle. Standard tamping machine arrangement The basic principles and functions of a tamping machine remain the same regardless of manufacturer with only minor differences in design. Drive The majority of track machines are powered by a diesel engine. This provides power to the driving wheels via either a hydrostatic circuit or cardan shaft, allowing the machine to propel itself to and around a work-site. The engine also drives a hydraulic pump to provide power for the various tools. Lifting lining unit The lifting lining unit of a tamping machine lifts and holds the track in corrected position while being tamped. All types of units require the following components to achieve this task: Chassis Normally rectangular with four wheels to ride on the rails Lifting cylinders One cylinder on either side connecting the unit to main machine chassis and allowing the unit to lift track