You are an expert at summarizing long articles. Proceed to summarize the following text:

You are an expert at summarizing long articles. Proceed to summarize the following text: 
BACKGROUND 
     Generally, railroad tracks include a pair of parallel rails coupled to a series of laterally extending ties (or sleepers). Ties may be made from concrete or wood. Each tie is coupled to the rails by metal tie plates and/or spring clips. The ties are disposed on a ballast bed. The ballast may be a hard particulate material, such as gravel. The ballast filled space between the ties is called a crib. 
     Although appearing rigid, rails are flexible members that can bend and distort, for example under the load of trains passing over. The ballast acts like a cushion absorbing some of the shock. Ballast can also help keep the rail level and allow moisture and rain water to drain away. 
     During installation and maintenance, ballast may be “tamped” to maintain proper position of the ties. Tamping involves agitating the ballast to allow the particles to re-position, and compact it under the tie. 
     A tamping device includes one or more workheads mounted on a motorized vehicle that travels on the rails. A workhead may include a pair of elongated, vertically extending tools structured to move together vertically and horizontally in a pincer-like motion. The workhead has two sets of tools spaced so that each tool may be disposed on opposite lateral sides of a rail. The workhead may further include a vibration device configured to rapidly vibrate the tools. 
     A tamping vehicle typically carries at least one operator. The vehicle accelerates under its own power to the ties requiring work. As it approaches the tie, it slows down, then stops at a tie and performs the required tamping work, and moves to the next tie to repeat the cycle. Tamping work may involve agitating and compacting the ballast through the vertical movement and vibration of the workhead. The constant movement and vibration of the tamping device may cause discomfort to operators sitting in tamping vehicles for extended periods of time. 
     BRIEF SUMMARY 
     In an example, a rail maintenance vehicle includes a frame, a workhead, an actuator, a pilot valve, and a throttling valve. The frame includes wheels that travel along rails. The actuator extends and retracts the workhead with respect to the frame. The pilot valve receives a fluid and controls the flow of the fluid to at least one output. The throttling valve adjusts a pressure of the fluid at an output relative to a pressure of the fluid at an input. The pilot valve and the throttle valve are coupled such that the fluid travels through the pilot valve and the throttling valve to cause the workhead to be extended or retracted. 
     In another example, a method of operating a maintenance vehicle includes: supplying fluid to a pilot valve; supplying the fluid to a throttling valve that adjusts a pressure of the fluid at an output relative to a pressure of the fluid at an input; and controlling an actuator that extends and retracts a workhead relative to a frame of the vehicle using the fluid exiting the throttling valve. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  shows a tamping machine rail vehicle where a vertical ride quality system can be implemented, according to an example embodiment. 
         FIG. 2  shows a seat assembly where a vertical ride quality system can be implemented, according to an example embodiment. 
         FIG. 3  is a schematic of a control system, according to an example embodiment. 
         FIG. 4  shows a side view of an aspect of a vertical ride quality system, according to an example embodiment. 
     
    
    
     DETAILED DESCRIPTION 
     Embodiments of a vertical ride quality system and related methods for reducing the vertical vibration in a rail vehicle are described. It is to be understood, however, that the following explanation is merely exemplary in describing the devices and methods of the present disclosure. Accordingly, any number of reasonable and foreseeable modifications, changes, and/or substitutions are contemplated without departing from the spirit and scope of the present disclosure. 
     In an embodiment, the vertical ride quality system is employed in a tamping machine rail vehicle, as illustrated in  FIG. 1 .  FIG. 1  shows a tamping vehicle  100  that includes a frame assembly  102 , a propulsion device  104 , a tamping device  106 , and a cabin  108 . Frame assembly  102  includes a plurality of rigid frame members and a plurality of wheels  109  that are configured to travel on the pair of rails  101 . Tamping vehicle  100  travels across a pair of rails  101 , disposed over a series of rail ties  103 . The rails  101  and series of ties  103  are disposed over a bed of ballast. The propulsion system  104  is configured to move tamping vehicle  100 . The tamping device  106  is configured to tamp rail ties  103 . 
     The tamping device  106  may include multiple workheads. In the side view of  FIG. 1 , one workhead can be viewed while another workhead is also included at an opposite side corresponding with the other rail. Any number of workheads (2, 4, etc) may be included. The tamping device  106  includes paddles  110  that are lowered into the ballast. The paddles  110  are vibrated by vibrators  114 . The paddles  110  may be actuated by an actuator, which may be hydraulic, to squeeze the paddles around the rail ties. The tamping device  106  is coupled to the frame assembly  102  via a subframe  116  and an actuator  118 . The actuator  118  is preferably a hydraulic actuator and is operable to lower the tamping device  106  such that the paddles  110  are inserted into the ballast where the squeezing and vibration action tamps the ballast. In a work cycle, the tamping vehicle  100  advances to position the tamping device  106  over a tie. The actuator  118  is actuated to lower the tamping device  106  to carry out the tamping of the ballast. Then, the actuator  118  is actuated to raise (and in some cases stow) the tamping device  106  for travel to the next tie. 
     Tamping vehicle  100  may also include a tracking device  112  that measures the general linear movement of the rail vehicle  100  along the rail track  101 . Cabin  108  may be structured such that it remains stationary relative to the frame assembly  102  as the rail vehicle  100  moves along the railroad track  101 . 
       FIG. 2  shows a seat assembly  120  located within cabin  108 . An operator may sit on the seat and control the tamping machine rail vehicle  100  from within the cabin during a tamping or other operation. To reduce movement or vibrations felt by an operator in the seat assembly  120 , the seat assembly  120  may include a suspension  122  such as a pneumatic or air bag suspension to dampen vibrations in the seat assembly  120 . The effect of the suspension  122  is limited in that it operates to lessen vibrations passing through to the seat assembly  120 . The suspension  122  does not lessen the stresses induced in the remainder of the tamping vehicle  100 . Vertical ride quality felt by an operator (e.g., vibration) as well as the stresses induced in the tamping vehicle  100  may be reduced by limiting the vertical impact at the source. 
     Referring to  FIG. 3 , a control system  200  for controlling the raising and lowering of the workheads (and thereby the impact induced on the operator and the tamping vehicle  100 ) is illustrated. Hydraulic fluid (or any other fluid such as engine oil) is supplied by a source  202 , for example, an accumulator or the output of a hydraulic pump. The hydraulic fluid is supplied to a pilot valve  204  via an orifice  206 . The pilot valve  204  may be an electronically controlled valve such as a solenoid valve controlled via control wires  208 . The pilot valve  204  is illustrated as a three way valve having an input orifice  206  and two outputs  210  and  212 . The pilot valve may selectively direct hydraulic fluid to the output  210 , the output  212  or neither output. Other configurations (for example two solenoid valves) may also be used. The outputs  210  and  212  may be respectively coupled to the throttling valves  214  and  216 . The throttling valves  214 / 216 , which may be gate valves, are operable to reduce a pressure of the hydraulic fluid such that a pressure at an output side  220 / 224  is lower than at an input side  218 / 222  based on the setting of the throttle valves  214 / 216 . The setting of the throttling valves  214 / 216  may be adjusted via a threaded stem coupled to the gate of the valve. 
     The output sides  220 / 224  of the throttling valves  214 / 216  may be coupled to a spool  232  of the main valve  230 . The hydraulic fluid from the throttling valves  214 / 216  controls the position of the spool  232 , which in turn sets the speed and direction of the actuator  240 . After passing the spool  232 , the hydraulic fluid may be supplied to the main valve  230  or may be directed to the return  238 . The main valve  230 , based upon the position of the spool  232 , directs hydraulic fluid from the source  202  to the actuator  240 , which causes the tamping workhead to raise or lower. Hydraulic fluid may return via return  238 . 
     In an example, the throttling valve  214  may be associated with the compression stroke of the actuator  240 , which may in turn correspond to raising the tamping workhead. The throttling valve  216  may be associated with the extension stroke of the actuator  240 , which may in turn correspond to lowering the tamping workhead. 
     In an example, the orifice  206  may be enlarged (for example 1 mm as compared to 0.8 mm) to provide a greater range of available pressures at an output side  220 / 224  of the throttling valves  214 / 216 . Conversely, a smaller orifice (for example 0.8 mm as compared to 1.0 mm) may be used to reduce an upper limit as to how fast the main valve  230  can change direction and stroke of the actuator  240 . The size of the orifice controls the speed at which the pilot valve  204  shifts, which in turn (after passing through the throttling valves) controls the speed at which the spool  232  shifts. Controlling the rate at which the spool  232  of the main valve  230  shifts reduces the acceleration of the actuator  240 . 
     In operation, the pilot valve  204  may select whether hydraulic fluid is directed to lift or lower the tamping workhead. The throttling valve  214 / 216 , based upon its setting, affects how quickly the spool in the main valve  230  moves by changing the pressure and/or flow rate of the hydraulic fluid reaching the spool. 
     The throttling valve  214 / 216  may reduce the impact induced in the tamping vehicle  100 , and by extension the operator, by controlling how quickly the tamping workhead is inserted into and removed from the ballast. In the example where two throttling valves are included, the up stroke and the down stroke may be individually controlled. This allows the advantage of adjusting the stroke that is causing the most induced impact while allowing faster action on the other stroke to improve workhead cycle time. For example, the pressure to the side of the spool associated with raising the workhead may be limited more than the pressure to the side of the spool associated with lowering the workhead. It will be appreciated that multiple throttling valves may be provided for each workhead. For example, if the vehicle includes four workheads, there may be eight throttling vales to independently control the up and down stroke of each workhead individually. 
       FIG. 4  is a side view of a portion of a vertical ride quality system  300 . Vertical ride quality system  300  may include a series of control valves that may provide a dampening effect to the vehicle and thereby also the seat assembly  120 . Vertical ride quality system  300  includes a pilot valve  302  and throttling valves  304  and  305 , which may be provided by a single twin valve. The system  300  is coupled to an engine and a main control valve  352  of a workhead. Pilot valve  302  receives oil pumped from the engine via an orifice  308  and causes the oil to flow through the throttling valve  304  (or  305  based on the setting of the pilot valve  302 ) and into the main control valve  352 . 
     The engine drives a pump that supplies oil (or another hydraulic fluid) to the main control valve  352 . In an embodiment, orifice  308  is connected to this pump, and oil is ported so that it goes through the pilot valve  302 , throttling valve  304 , main control valve  352  and back into the engine spool. Oil delivered through orifice  308  controls the movement and speed of a spool of the main control valve  352  thereby controlling the acceleration of an actuator coupled to the main control valve  352 , which in turn controls impacts and vibrations experienced by the vehicle, and thus also the movement of seat assembly  120 . By controlling the flow of the oil, valves  302 ,  304 , and  352  collectively dampen the actuator and by extension the vibration of seat assembly  120 . Throttling valves  304  and  305  control the speed of the spool that shifts directions of the main control valve  352 . Throttling valves  304 / 305  may be of a gate valve type that include a knob and a faucet. The main control valve  352  controls the vertical movement of a workhead of tamping device  106 . 
     In an embodiment, manifold  360  includes one or more main control valves  352  for each of the one or more workheads of tamping device  106 . For example, the manifold  350  may include four main control valves  352  respectively associated with four workheads. 
     In an embodiment, the engine drives a pump that supplies oil to manifold  360 . Orifice  308  is connected to this pump, and oil is ported so that it goes through the pilot valve  302 , throttling valve  304 / 305  (which also may be included as a part of the manifold  360 ), main control valve  352  and back into the engine spool. The throttling valve  304 / 305  controls how fast the spool in the main control moves and the amount of oil that respectively flows into that one of the four main valves  352 . 
     In an embodiment, system  300  includes two throttling valves, valves  304  and  305 . The pair of throttling valves may be manually and individually adjustable. In an embodiment, a first throttling valve, for example valve  304 , may affect the down stroke of one or more workheads, and a second throttling valve, for example, valve  305 , may affect the up stroke of one or more workheads. In an embodiment, the twin throttle valves can be adjusted to avoid significantly prolonging the workhead cycle yet still provide dampening of system  300 . In this manner, the pair of throttling valves  304  and  305  can be adjusted by an operator to control the shifting time of the pilot valve  302  and the main control valve  352 . 
     It will be appreciated that twin throttling valves are not required. For example, a single throttling valve may be used in place of the twin throttle valves. The throttling valves may also be disposed at different locations in the system. For example, the throttling valve may also be placed before the pilot valve. 
     It will also be appreciated that this disclosure is not limited to rail vehicles that carry an operator. For example, an autonomous or drone vehicle can also realize advantages of the present disclosure such as reduced mechanical stresses on various parts of the vehicle. 
     The breadth and scope of the present disclosure should not be limited by any of the above-described exemplary embodiments, but should be defined only in accordance with the following claims and their equivalents. Moreover, the above advantages and features are provided in described embodiments, but shall not limit the application of the claims to processes and structures accomplishing any or all of the above advantages. 
     Additionally, the section headings herein are provided for consistency with the suggestions under 37 CFR 1.77 or otherwise to provide organizational cues. These headings shall not limit or characterize the invention(s) set out in any claims that may issue from this disclosure. Further, a description of a technology in the “Background” is not to be construed as an admission that technology is prior art to any invention(s) in this disclosure. Neither is the “Brief Summary” to be considered as a characterization of the invention(s) set forth in the claims found herein. Furthermore, any reference in this disclosure to “invention” in the singular should not be used to argue that there is only a single point of novelty claimed in this disclosure. Multiple inventions may be set forth according to the limitations of the multiple claims associated with this disclosure, and the claims accordingly define the invention(s), and their equivalents, that are protected thereby. In all instances, the scope of the claims shall be considered on their own merits in light of the specification, but should not be constrained by the headings set forth herein.

Summary:
A rail maintenance vehicle includes a frame, a workhead, an actuator, a pilot valve, and a throttling valve. The frame includes wheels that travel along rails. The actuator extends and retracts the workhead with respect to the frame. The pilot valve receives a fluid and controls the flow of the fluid to at least one output. The throttling valve adjusts a pressure of the fluid at an output relative to a pressure of the fluid at an input. The pilot valve and the throttle valve are coupled such that the fluid travels through the pilot valve and the throttling valve to cause the workhead to be extended or retracted.