Abstract:
A track traveling vehicle having rubber tires for engaging a public road to permit the vehicle to travel thereon, and also having metal wheels capable of engaging rails associated with a track for permitting the vehicle to travel therealong. The metal wheels are supported on the chassis of the vehicle for vertical movement between a raised position wherein the rubber tires engage the road, and a lowered position wherein the metal wheels engage the rails and the rubber tires are maintained in an upwardly suspended non-engaged position. Hydraulic pumps are driven by the rubber tires when they are in the raised suspended position, and the hydraulic pumps supply hydraulic fluid to hydraulic motors to effect driving thereof. The hydraulic motors are in turn drivingly coupled to either the front or rear metal wheels to effect driving thereof when the metal wheels are engaged on the rails so that the vehicle can travel therealong.

Description:
FIELD OF THE INVENTION 
     The invention relates to a driving mechanism of a track traveling vehicle having rubber tires and iron wheels on its chassis and which is capable of self-propelling on a track of a rail road as well as on a public road, wherein the iron wheels can be turned by a driving force of the rubber tires. 
     BACKGROUND OF THE INVENTION 
     In a track provided to allow a street car or train to travel thereon, periodical operations are frequently conducted. There are a variety of operations as periodical operations, such as maintenance and inspection of a trolley line that is constructed over the track in addition to ground operations such as tamping of smashed stones paved on the track or change of tiles. In such operations relating to the track, operators and materials need to be moved to an operating or job site, and most job sites normally have no road or traffic access. In many cases a track traveling vehicle (also known as a land track traveling vehicle) capable of traveling on both a public road and a track is used to move operators or materials to an inconveniently situated job site. 
     The foregoing track traveling vehicle has a body or chassis of a generally known truck provided with rubber tires as its base, and iron wheels provided under the chassis to travel on a track. The track traveling vehicle travels on a public or normal road by rubber tires so as to move to a railway crossing close to a job site so as to move from a standby location in an urban district area to the job site located in the suburbs and is positioned along the track. If the track traveling vehicle enters the railway crossing, it is lifted by hydraulic jacks or the like to be turned at right angles with the road so as to align the track traveling vehicle with the direction of the track. Thereafter, the iron wheels are lowered to contact the rails while the lower ends of the rubber tires are forced to float in the air. In this state, the track traveling vehicle is held by the respective iron wheels so that the track traveling vehicle is moved along the track when the iron wheels are driven. In the track traveling vehicle capable of travelling on both the public road and the track, it can travel on the public road at high speed until it reaches a railway crossing from a standby location of a maintenance or construction company, and can also travel on rails from the railway crossing to a job site, thereby quickly and efficiently moving operators and materials to the job site. 
     There have been conventionally employed two types of methods for driving a track traveling vehicle having the foregoing construction on rails. The first method is to lift a chassis by iron wheels while the rubber tires of the truck are maintained in contact with the surfaces of rails. In this method, if the rubber tires are turned to travel on a general public road, a driving force or tuning force of the rubber tires moves the chassis owing to frictional force generated between the rubber tires and the rails. At this time, the iron wheels have the function to merely guide the chassis not to come out from the rails, while the motion of the chassis depends on the frictional force of the rubber tires. In the first method, the rubber tires are forced to contact the rails while keeping an appropriate pressure therebetween, resulting in a difficulty in controlling thereof. 
     The second method is to lift the entire chassis by the iron wheels while the rubber tires are forced to float in the air, then the iron wheels are driven by hydraulic motors connected to the iron wheels. In this method, the speed (i.e. rpm) and turning direction can be controlled by the amount of a hydraulic oil supplied to the hydraulic motors, resulting in a characteristic of easy driving of the track traveling vehicle. There is conventionally employed two ways for sucking and discharging the hydraulic oil to the hydraulic motors. One way is to apply power from a vehicle battery to electric motors, so that hydraulic pumps are driven by the electric motors to discharge hydraulic oil. This way has, however, a drawback in that a large amount of hydraulic oil is not discharged by the power applied by the battery, and energy consumption in the battery is large. The second way is to connect hydraulic pumps to an engine mounted on the chassis and hydraulic oil is discharged from the hydraulic pumps by the turning force of the engine. In the second way, the tuning or driving force of the engine can be directly transmitted to the hydraulic pumps as it is very efficient, and hence this way is widely employed. 
     The conventional track traveling vehicle has a mechanism wherein hydraulic pumps are directly connected to an output shaft of an engine, and hydraulic motors are driven by hydraulic oil discharged by the hydraulic pumps, and an output of the hydraulic motors drives or turn the iron wheels. This mechanism has not employed fixed discharge amount hydraulic pumps, but employs variable discharge amount hydraulic pumps (for example as disclosed in Japanese Patent Laid-Open Publication No. 11-189155). These variable discharge amount hydraulic pumps function to control the amount of hydraulic oil that is discharged by controlling a control pressure even if the output of the engine is constant. If variable discharge amount hydraulic pumps are used, the discharge amount of hydraulic oil can be varied by stages from zero to the maximum by controlling a control pressure so that the hydraulic motors, namely, the rpm of the iron wheels can be varied by stages by controlling the amount of discharge of hydraulic oil from the hydraulic pumps. In such a control method, there is an advantage that the rpm of the iron wheels can be varied smoothly from zero to the maximum, and also the controlling operation is easily made to smoothly drive the track traveling vehicle. However, the variable discharge amount hydraulic pumps are expensive in cost and complex in hydraulic system piping for controlling purposes. Further, since the variable discharge amount hydraulic pumps are fixed to the chassis side, and the hydraulic motors for driving the iron wheels are fixed to the rear portion of the chassis, there is a drawback that the length of hydraulic piping for connecting the variable discharge amount hydraulic pumps and the hydraulic motors becomes long because these pumps and the hydraulic motors are connected by a hydraulic hose or the like. Accordingly, the conventional variable discharge amount hydraulic pumps have many drawbacks in view of cost and design thereof although they are excellent in operability. 
     SUMMARY OF THE INVENTION 
     The driving mechanism of a track traveling vehicle of the invention is characterized in a construction that hydraulic pumps are driven by rubber tires of a generally known truck that forms a base of the track traveling vehicle, and hydraulic motors are driven by hydraulic oil discharged by the hydraulic pumps. With this construction, a driving force between an engine and a transmission is transmitted to the rubber tires to operate the hydraulic pumps as it is without altering or reforming the driving mechanism of the truck. It is possible to suck and discharge hydraulic oil in the same manner as in an ordinary truck, and the turning output of the engine is changed to effect forward and backward operations or stopping operation. Further, since the driving mechanism of a track traveling vehicle can employ cheaper fixed discharge amount hydraulic pumps without using expensive variable discharge amount hydraulic pumps, the mechanism becomes cheaper in cost. 
     To achieve the above objects, a driving mechanism of a track traveling vehicle according to a first aspect of the invention capable of traveling on both a road and a track comprises front and rear rubber (i.e. road) tires supported by a chassis at the lower portion thereof, front and rear iron (i.e. track) wheels supported by the chassis and movable vertically, hydraulic pumps and hydraulic motors, wherein the track traveling vehicle travels on the road when the iron wheel are lifted while the front and rear rubber tires are forced to contact the ground, and the track traveling vehicle travels on the track when the front and rear iron wheels are lowered to contact rails while the front and rear rubber tires are forced to float in the air (i.e., in a raised suspended state out of engagement with either the road or track), and wherein the hydraulic pumps are driven by the rear rubber tires so as to suck and discharge hydraulic oil to the hydraulic motors, and the front or rear iron wheels are driven by the hydraulic motors so that the track traveling vehicle travels on the track. 
     The driving mechanism of a track traveling vehicle according to a second aspect of the invention is characterized in that the driving mechanism of the first aspect of the invention comprises a swing portion fixed to the chassis and arranged to be vertically swung by hydraulic cylinders, friction wheels supported by the swing portion and capable of contacting rear rubber tires at the peripheries thereof when the swing portion is turned, and the hydraulic pumps being driven when the friction wheels are turned. 
     The driving mechanism of a track traveling vehicle according to a third aspect of the invention is characterized in that the driving mechanism of the first aspect of the invention comprises a swing portion fixed to the chassis and arranged to be vertically swung by hydraulic cylinders, an axle supporting portion fixed to the swing portion at the lower portion thereof, the rear iron wheels supported by the axle supporting portion at both sides thereof, and the hydraulic motors provided on the axle supporting portion for driving the rear iron wheels, wherein the swing portion and the axle supporting portion are turned when the hydraulic cylinders are operated so that the rear iron wheels supported by the axle supporting portion are forced to contact rails to allow the rubber tires to float in the air. 
     The driving mechanism of a track traveling vehicle according to a fourth aspect of the invention is characterized in that the driving mechanism of the first aspect of the invention comprises a swing portion fixed to the chassis and arranged to be vertically swung by hydraulic cylinders, and an axle supporting portion fixed to the swing portion at the lower portion, wherein the axle supporting portion is connected to the swing portion so as to be vertically laterally swung (i.e. to the left and right). 
     A driving mechanism of a track traveling vehicle according to a fifth aspect of the invention capable of traveling on both a road and a track, comprising front and rear rubber tires supported by a chassis at the lower portion thereof, front and rear iron wheels supported by the chassis and movable vertically, wherein the track traveling vehicle travels on the road when the iron wheel are lifted while the front and rear rubber tires are forced to contact the ground, and the track traveling vehicle travels on the track when the front and rear iron wheels are lowered to contact rails while the front and rear rubber tires are forced to float in the air, wherein said driving mechanism further comprises hydraulic pumps for driving use that are driven when the rear rubber tires are turned, hydraulic motors that are driven by hydraulic oil discharged by the hydraulic pumps, and a hydraulic pump for oil supply use that is driven when the rear rubber tires are turned, wherein two hydraulic circulation circuits are formed by connecting and closing both hydraulic output ends of the hydraulic pumps for driving use and the hydraulic motors, and wherein a hydraulic output end of the hydraulic pump for oil supply use at the discharge side communicates with the hydraulic circulation circuits, so that two hydraulic pumps are driven at the same time when the rear rubber tires are turned to discharge the hydraulic oil, thereby supplying the hydraulic oil that is discharged from the hydraulic pump for oil supply use to the hydraulic oil circulation circuits. 
     The driving mechanism of a track traveling vehicle according to a sixth aspect of the invention is characterized in that in the fifth aspect of the invention, the number of hydraulic pumps for driving use is two, and the number of hydraulic motors for turning the rear iron wheels is two, and the number of the hydraulic pump for oil supply use is one, wherein the three hydraulic pumps are operated at the same time when the rear rubber tires are turned, and wherein the hydraulic pumps for driving use are connected to both hydraulic output ends of the hydraulic motors to form two hydraulic circulation circuits, the hydraulic output end of the hydraulic pump for oil supply use at the discharge side communicates with the hydraulic circulation circuits, and wherein hydraulic oil discharged from the hydraulic pump for oil supply use is supplied to the respective independent hydraulic circulation circuits for turning the hydraulic motors. 
     The driving mechanism of a track traveling vehicle according to a seventh aspect of the invention is characterized in that the hydraulic circulation circuits of the fifth or sixth aspect of the invention are formed by connecting and closing both hydraulic output ends of the hydraulic pumps for driving use and hydraulic motors for turning the rear iron wheels, and a safety circuit is provided between pressure application sides and application collection sides of the hydraulic circulation circuits for leaking hydraulic oil of one hydraulic circulation circuit to the other hydraulic circulation circuit and discharging overflowed hydraulic oil when hydraulic oil exceeds a prescribed value. 
     The driving mechanism of a track traveling vehicle according to an eighth aspect of the invention is characterized in that in the fifth or sixth aspect of the invention, both hydraulic output ends of the hydraulic pump for oil supply use communicates with an oil tank via check valves directed in a forward direction, and also communicates with the oil tank via relief valves directed in a forward direction, and is further connected to both hydraulic output ends of the hydraulic pumps for driving use via check valves directed in a forward direction. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a perspective view showing a state where a track traveling vehicle according to a first embodiment of the invention is placed and travels on a track; 
     FIG. 2 is a side view of the track traveling vehicle in FIG. 1; 
     FIG. 3 is a rear view of the track traveling vehicle in FIG. 1; 
     FIG. 4 is an enlarged perspective view of one of front jack mechanisms for vertically moving a front iron wheel of the track traveling vehicle according to the first embodiment of the invention as viewed from a slanting front side thereof; 
     FIG. 5 is a sectional view of the front jack mechanism shown in FIG. 4 cut in a longitudinal direction; 
     FIG. 6 is an exploded perspective view of parts of a driving mechanism of a track travelling vehicle for vertically moving rear iron wheels and transmitting a rotating force or a turning effort of the rear tires to the rear iron wheels; 
     FIG. 7 is a side elevational view of the driving mechanism of a track travelling vehicle according to the first embodiment of the invention for vertically moving rear iron wheels and transmitting a turning effort of the rear tires to the rear iron wheels; 
     FIG. 8 is a hydraulic circuit showing generation of a hydraulic pressure and flow paths of hydraulic oil for turning the rear iron wheels in the driving mechanism of a track traveling vehicle according to the first embodiment of the invention; 
     FIG. 9 is an exploded perspective view of disassembled parts of a mechanism fixed to the rear portion of a chassis for vertically moving rear iron wheels and generating a hydraulic pressure serving as a power transmitted from the rear tires to the rear iron wheels in a driving mechanism of a track traveling vehicle according to a second embodiment of the invention; 
     FIG. 10 is an exploded perspective view of a gear box for transmitting a turning effort from rear tires to hydraulic motors in the driving mechanism of a track traveling vehicle according to the second embodiment of the invention; 
     FIG. 11 is an enlarged perspective view of one of front jack mechanisms for vertically moving a front iron wheel of the track traveling vehicle according to the second embodiment of the invention as viewed from a slanting front side thereof; 
     FIG. 12 is an exploded perspective view of a mechanism supported by one of the front jack mechanisms for driving the front iron wheel in the driving mechanism of a track traveling vehicle mechanism according to the second embodiment of the invention; 
     FIG. 13 is a hydraulic circuit showing generation of a hydraulic pressure and flow paths of a hydraulic oil for turning the front iron wheels in the driving mechanism of a track traveling vehicle according to the second embodiment of the invention; and 
     FIG. 14 is a view showing flow of hydraulic oil in the driving mechanism of a track traveling vehicle according to the invention. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     FIRST EMBODIMENT 
     FIGS. 1 to  8  and  14   
     A driving mechanism of a track traveling vehicle according to the first embodiment is described with reference to FIGS. 1 to  8  and  14 . 
     FIG. 1 is a perspective view of a track traveling vehicle  10  according to a first embodiment of the invention, and it shows a state where the track traveling vehicle  10  is placed and travels on parallel rails  28 ,  29  provided on a track. FIG. 2 is a side view of the track traveling vehicle  10  in a state where it travels on the track, and FIG. 3 is a rear view of the track traveling vehicle  10  in FIG.  1 . In these figures, the track is formed by a pair of rails  28 ,  29  which are disposed at a given spaced interval, and the rails  28 ,  29  are fixed onto railroad ties  30  which are disposed in given intervals on broken or smashed stones or the like. 
     A main body of the track traveling vehicle  10  is a conventional truck provided with an engine, and it represents a body construction of a so-called dump truck. A chassis  11  for supporting the entire track traveling vehicle  10  is formed of a frame prepared by welding channel-shaped steel members. Front rubber road tires  12  are supported by front left and right sides of the chassis  11 , and left rear rubber road tires  13 ,  14 , and right rear rubber road tires  15 ,  16  are respectively supported as double tires by the rear left and right sides thereof. A cabin  17  in which a driver operates the track traveling vehicle  10  is fixed onto the chassis  11  at the front end over the front tires  12 . A rear body  18  having a box shape which is loaded with gravel or earth and sand is fixed onto the rear portion of the chassis  11 . The constructions of the chassis  11 , front tires  12 , left rear tires  13 ,  14 , right rear tires  15 ,  16 , cabin  17 , and rear body  18  are the same as those of a generally conventionally known truck, whereby the track traveling vehicle  10  is assembled by reforming a conventional, commercially available truck. 
     Front jack mechanisms  21  are fixed onto the chassis  11  at the left and right sides between the cabin  17  and rear body  18  (only the left side front jack mechanism  21  is shown in FIG. 1 and 2, a right side front jack mechanism  21  opposite to the left side front jack mechanism  21  is not shown in FIGS. 1 and 2 for convenience, but the right side front jack mechanism  21  is symmetrical with the left side front jack mechanism  21 ). Front iron or metal wheels  22  are supported by the front jack mechanisms  21  at the lower portions thereof. Since the front jack mechanisms  21  have hydraulic cylinders therein, they can be vertically telescoped in the longitudinal direction thereof, thereby vertically moving the front iron wheels  22 . When the front iron wheel  22  is lowered by its respective hydraulic cylinder, it can contact the respective rail  28 ,  29 . 
     A driving mechanism  24  which can be turned by hydraulic cylinders  88 ,  89  is fixed to the chassis  11  at the rear portion thereof wherein the track traveling vehicle  10  can be moved on the track by the driving mechanism  24 . Rear iron or metal wheels  25 ,  26  are supported by the lower portion of the driving mechanism  24  at the left and right sides, wherein when the driving mechanism  24  is turned by the hydraulic cylinders  88 ,  89 , the rear iron wheel  25  contacts the rail  28  while the rear iron wheel  26  contacts the rail  29 , thereby lifting the chassis  11 . With the construction, when the track traveling vehicle  10  travels on a public road, the front iron wheels  22  are pulled up by the front jack mechanisms  21  and the rear iron wheels  25 ,  26  are also pulled up by the driving mechanism  24 . As a result, since the front tires  12 , left rear tires  13 ,  14  and right rear tires  15 ,  16  contact the ground, the output from an engine of the track traveling vehicle  10  is transmitted to the left rear tires  13 ,  14  and right rear tires  15 ,  16  so that the entire track traveling vehicle  10  is forced to travel on the public road. When the direction of the track traveling vehicle  10  is turned to different directions, the track traveling vehicle  10  can be moved to an intended direction by operating the front tires  12  to the left and right. Such a function is the same as that of a conventionally known truck. 
     Next, when the track traveling vehicle  10  is forced to travel on the track, it is driven on a public road until it reaches a railway crossing where the track crosses the public road. Thereafter, the track traveling vehicle  10  is forced to stop at the center of the railway crossing, and a conventional jack-up mechanism (generally fixed to the central lower side of the chassis  11 ), not shown, is operated to lift the entire track traveling vehicle  10 , then the track traveling vehicle  10  is rotated through a right angle. In consequence, the moving direction of the track traveling vehicle  10  becomes parallel with the extension direction of the track. At this position, when the front iron wheels  22  are lowered by the front jack mechanisms  21 , the front iron wheels  22  contact the rails  28 ,  29  while when the driving mechanism  24  is turned, the rear iron wheel  25  contacts the rail  28  and the rear iron wheel  26  contacts the rail  29 . Thereafter, when the jack-up mechanism is contracted, the entire chassis  11  is supported by the front iron wheels  22  and rear iron wheels  25 ,  26  so that the front tires  12 , left rear tires  13 ,  14  and right rear tires  15 ,  16  are spaced upwardly away from the rails  28 ,  29 . When the engine of the track traveling vehicle  10  is driven in a state where the left rear tires  13 ,  14  and right rear tires  15 ,  16  are lifted or hoisted, the driving force of the engine is transmitted to the left rear tires  13 ,  14  and right rear tires  15 ,  16 . When the driving mechanism  24  is turned, friction wheels  72 ,  73  supported by the driving mechanism  24  are brought into contact with and pressed against the rear tires  14 ,  15 , so that the rotation or turning of the rear tires  14 ,  15  is transmitted to the friction wheels  72 ,  73 . When the friction wheels  72 ,  73  are turned, hydraulic pumps  69 ,  70 ,  71  are driven so that they suck hydraulic oil and discharge it to hydraulic motors  98 ,  99  connected to the rear iron wheels  25 ,  26  to drive the hydraulic motors  98 ,  99 . When the rear iron wheels  25 ,  26  are turned by the hydraulic motors  98 ,  99 , the track traveling vehicle  10  can move on the rails  28 ,  29  serving as the track. 
     The principle for driving the rear iron wheels  25 ,  26  by the turning of the friction wheels  72 ,  73  is described with reference to FIG.  14 . FIG. 14 shows a summary of a hydraulic system of the driving mechanism of a track traveling vehicle according to the invention. Although the combination of the left rear tire  14  and the friction wheel  72  is shown in FIG. 14, a driving mechanism of the rear iron wheel  26  by the combination of the right rear tire  15  and the friction wheel  73  has the same construction as the combination of left rear tire  14  and friction wheel  72 , and hence it is omitted in FIG.  14 . 
     In FIG. 14, the driving mechanism  24  is turned so that the outer periphery of the friction wheel  72  is brought into contact with that of the left rear tire  14 . The friction wheel  72  is fixed to a common axle  68  and the common axle  68  is rotatably or turnably supported by the driving mechanism  24 . A hydraulic pump  69  is connected to the common axle  68  via a gear box  111 . A hydraulic motor  98  is built in the driving mechanism  24 , and a turning effort of the hydraulic motor  98  is transmitted to the rear iron wheel  25  via gear box  114 . Both hydraulic output ends of the hydraulic pump  69  are connected to both hydraulic output ends of the hydraulic motor  98  wherein the hydraulic pump  69  and the hydraulic motor  98  form a closed circulation circuit. With the construction, when the rear tire  14  is turned, the friction wheel  72  is turned owing to a frictional force, thereby turning the common axle  68  that is turned together with the friction wheel  72 , so that the hydraulic pump  69  is driven via the gear box  111 . Accordingly, hydraulic oil is discharged from the hydraulic pump  69  and is supplied to the hydraulic motor  98 , thereby turning the hydraulic motor  98 . The turning or rpm of the hydraulic motor  98  is reduced by the gear box  114  to turn the rear iron wheel  25  so as to generate a motive power for moving the track traveling vehicle  10  on the rail  28 . In such a route, the turning effort of the left rear tire  14  is transmitted to the rear iron wheel  25  so that the speed and the forward (or normal) and reversal turning of the rear iron wheel  25  are adjusted by controlling the driving rotation of the left rear tire  14 . As a result, the track traveling vehicle  10  can be moved on the rails  28 ,  29  in the same sense or feeling as the operation of the conventional truck. 
     An oil supply circuit  119  (FIG. 14) is provided in this hydraulic driving system. The oil supply circuit  119  always supplies hydraulic oil to the closed circuit formed by connecting both hydraulic output ends of the hydraulic pump  69  and those of hydraulic motor  98 , thereby preventing the closed circuit from being short of hydraulic oil. The oil supply circuit  119  mainly comprises a hydraulic pump  70  and an oil tank  131  filled with hydraulic oil. The hydraulic pump  70  is connected to the common axle  68  via a gear box  112  so that the hydraulic pump  70  is driven together with the hydraulic pump  69  in the same direction. The oil tank  131  communicates with both hydraulic output ends of the hydraulic pump  70  and both hydraulic output ends of the hydraulic motor  98  via check valves  132 ,  133  respectively provided in the reverse or backflow direction, while the oil tank  113  also communicate with both hydraulic output ends of the hydraulic pump  70  via relief valves  136 ,  137 . Both hydraulic output ends of the hydraulic pump  69  through which hydraulic oil is sucked and discharged are connected to both hydraulic output ends of the hydraulic pump  70  via check valves  134 ,  135  provided in the forward direction. That is, the hydraulic oil discharged from the hydraulic pump  70  is always supplied to hydraulic pipes connecting between the hydraulic pump  69  and hydraulic motor  98 . 
     In the oil supply circuit  119 , when the common axle  68  is turned by the left rear tire  14 , the hydraulic pump  69  is driven via the gear box  111  so that the hydraulic pump  69  sucks the hydraulic oil stored in the oil tank  113 . When the hydraulic pump  69  is turned in one direction, the hydraulic oil sucked by the check valve  132  passes through the hydraulic pump  70  and by the check valve  134 , then it is supplied to one end of a hydraulic output end (to the side for discharging the hydraulic oil at that time) of the hydraulic pump  69 . When the hydraulic pump  69  is reversely turned, the hydraulic oil sucked by the check valve  133  passes through the hydraulic pump  70  and through check valve  135  and is supplied to the other end (to the side for discharging the hydraulic oil at that time, and it is opposite to the forgoing side because the hydraulic pump  69  is reversely turned) of the hydraulic pump  69 . Since the hydraulic pressure generated in the hydraulic pump  70  is always applied to the discharge side of the hydraulic pump  69 , the closed hydraulic circuits formed by the hydraulic pump  69  and the hydraulic motor  98  are always filled with hydraulic oil. Accordingly, it is possible to prevent a phenomenon of non-transmittance of the driving force caused by a shortage of hydraulic oil, thereby driving the hydraulic motor  98  with assurance. The hydraulic oil which is discharged from the hydraulic pump  70  and not supplied to the hydraulic pump  69 , namely, any excess of hydraulic oil, passes through either the relief valve  136  or relief valve  137  and is returned to the oil tank  113 . 
     With such a hydraulic mechanism, when the left rear tire  14  is turned by an engine of the track traveling vehicle  10 , the hydraulic pump  69  discharges the hydraulic oil by the turning effort of the left rear tire  14 , and the thus discharged hydraulic oil drives the hydraulic motor  98  to turn the rear iron wheel  25  via the gear box  114 . At the same time, the hydraulic pump  70  is also driven to supply the hydraulic oil to the closed circuit formed by the hydraulic pump  69  and hydraulic motor  98 , thereby preventing the hydraulic pump  69  from being idly turned. The hydraulic mechanism is not directly connected to the engine of the track traveling vehicle  10 , and the rear iron wheel  25  is turned by the driving force of the left rear tire  14 , thereby achieving a cheaper mechanism because expensive variable discharge amount hydraulic pumps are not used. Since the hydraulic mechanism can be constructed separately from the truck, it is not necessary to reform or rebuild the truck to a large extent so that it can be easily assembled. 
     FIGS. 4 and 5 describe the front jack mechanism  21  more in detail, wherein FIG. 4 is the perspective view of the front jack mechanism  21  and FIG. 5 is a sectional view of the front jack mechanism  21  which is cut in the longitudinal direction. 
     The front jack mechanism  21  is assembled as one unit and each front iron wheel  22  is supported by the front jack mechanism  21  at the lower portion thereof, whereby the front iron wheel  22  can be vertically moved relative to the chassis  11 . A long flat attachment plate  41  is fixed in the vertical direction to the outer surface of the chassis  11  formed by a channel-shaped steel member. An attachment plate  42  is fixed to the side surface of the attachment plate  41  at the upper portion thereof in the outward direction, and an outer upright tube  43  is fixed to the attachment plate  42 . The outer tube  43  is a square box-shape in cross section, and is closed at the upper end while opened at the lower end. The outer tube  43  is fixed to the attachment plate  42  in the manner that an axial line thereof is perpendicular to the chassis  11 . An inner tube  44  is vertically slidably inserted into the outer tube  43  from the lower end opening thereof. The inner tube  44  has a square shape in cross section. A closing plate  49  formed by a square thin plate is brought into intimate contact with and closes the lower end opening of the inner tube  44 . 
     A fluid pressure (i.e. hydraulic) cylinder  47  is housed in the rectangular parallelepiped inner space formed by the outer tube  43  and inner tube  44 . The base portion of the hydraulic cylinder  47  is fixed to a ceiling plate of the outer tube  43 , and the longitudinal direction of the hydraulic cylinder  47  is aligned with that of the outer tube  43 . A cylinder rod  48  telescopically protrudes from the lower end of the hydraulic cylinder  47 , and the lower end of the cylinder rod  48  is connected to the upper surface of the closing plate  49 . When hydraulic oil is supplied to the hydraulic cylinder  47 , the cylinder rod  48  is vertically telescoped, and the closing plate  49  connected to the cylinder rod  48  is vertically moved so that the inner tube  44  is also vertically moved. As a result, the inner tube  44  is vertically slid from the lower end opening of the outer tube  43  to perform a telescopic operation. A pair of axle supporting plates  50 ,  51  are fixed to the left and right sides of the closing plate  49  and arranged in parallel with each other, and they are directed downward. The front iron wheel  22  is inserted between the axle supporting plates  50 ,  51  and turnably supported by an axle shaft  52  that is inserted between the axle supporting plates  50 ,  51  in a lateral direction. A slider  46  is fixed to the side surface of the axle supporting plate  51  in the direction of the chassis  11  and it is slidably engaged with a guide plate  45  that is fixed to the side surface of the attachment plate  41  in the vertical direction. Accordingly, the slider  46  is slid by the guide plate  45  and guided only vertically. 
     Since the front jack mechanism  21  has such a construction, when the hydraulic oil is supplied to the hydraulic cylinder  47 , the cylinder rod  48  telescopes vertically, thereby moving the inner tube  44 , closing plate  49 , axle supporting plates  50 ,  51  respectively vertically. As a result, when the front iron wheel  22  is moved vertically, the lower end of the front iron wheel  22  is moved downwardly into contact with the rail  28  or moved upwardly away from the rail  28 . 
     FIGS. 6 and 7 describe the driving mechanism  24  more in detail wherein FIG. 6 is an exploded perspective view of disassembled parts of the driving mechanism  24 , and FIG. 7 is a side view showing a state where the parts shown in FIG. 6 are assembled and fixed to the rear portion of the chassis  11 . The driving mechanism  24  is formed of one driving unit and it is assembled so as to be fixed to the track traveling vehicle  10  even if a conventional truck is not reformed to a large extent. The track traveling vehicle  10  can be moved back and forth on the rails  28 ,  29  by the driving mechanism  24  serving as a main constituent of the invention. The driving mechanism  24  mainly comprises a swing portion  61  and an axle portion  62 . The swing portion  61  is vertically swingably connected to the rear portion of the chassis  11  while the axle portion  62  is laterally swingably connected to the lower portion of the swing portion  61 . 
     The swing portion  61  comprises a pair of operating or end plates  64 ,  65  and a connecting body  66  forming a construction that is opened in a U-shape directed upwardly. The end plates  64 ,  65  and connecting body  66  can be fixed to the chassis  11  at the rear portion as a unit. The end plates  64 ,  65  are formed by cutting steel plates and have respectively a substantially L-shape as viewed from the side thereof, and they have respectively a triangle between the center of the long side and tip ends of the short side. The end plates  64 ,  65  are respectively arranged in the manner that each long side thereof is substantially directed vertically while each short side is substantially directed horizontally. Subsequently, the connecting body  66  is formed of a channel member that is square in cross section and hollow in the internal thereof. The connecting body  66  is disposed horizontally and the lower surfaces of the end plates  64 ,  65  at the short side thereof are connected to both ends of the connecting body  66 . A pair of axle supporting plates  74  which extend downward and confront one another are fixed to the central opposite sides of the connecting body  66 . The U-shaped construction of the entire swing portion  61  is formed by this construction, and various members are fixed to this U-shaped construction. 
     The common axle  68  is rotatably supported by substantially L-shaped corner portions of the end plates  64 ,  65 . The common axle  68  and the connecting body  66  are arranged in the manner that the axial line of the common axle  68  is in parallel with the longitudinal direction of the connecting body  66 , and both ends of the common axle  68  protrude outward from the end plates  64 ,  65 . The hydraulic pumps  69 ,  70 ,  71  are disposed on the periphery of the common axle  68  wherein the hydraulic pump  69  is fixed to the end plate  64  at the outer side thereof while the hydraulic pump  71  is fixed to the end plate  65  at the outer side thereof and the hydraulic pump  70  is fixed between end plates  64 ,  65 . The friction wheels  72 ,  73  are fixed to the opposite ends of the common axle  68  wherein the friction wheel  72  is forced to contact the left rear tire  14  while the friction wheel  73  is forced to contact the right rear tire  15 . The upper end of the end plate  64  at the long side is perforated to form a pin hole  78  while the upper end of the end plate  65  at the long side is perforated to form a pin hole  79 . The middle portion of the end plate  64  at the long side is perforated to form an axle hole  76  while the middle portion of the end plate  65  is perforated to form an axle hole  77 . 
     Fixed axle plates  81 ,  82  are respectively fixed to the chassis  11  to rotatably support the swing portion  61 , and they have respectively a configuration of a notchlike claw of a crab that is opened at right angles as viewed from the side surface thereof. The fixed axle plates  81 ,  82  are respectively perforated at the base thereof to form holes through which pins  83 ,  84  are inserted. The pair of fixed axle plates  81 ,  81  are provided in the vicinity of both side surfaces of the end plate  64  whereby the end plate  64  is turnably supported by the pin  83  when the pin  83  is inserted into the holes of the fixed axle plates  81  and the axle hole  76  of the end plate  64 . Likewise, the pair of fixed axle plates  82 ,  82  are provided in the vicinity of both side surfaces of the end plate  65  whereby the end plate  65  is turnably supported by the pin  84  when the pin  84  is inserted into the holes of the fixed axle plates  82  and the axle hole  77  of the end plate  65 . 
     The pair of pressure (i.e. hydraulic) cylinders  88 ,  89  are respectively held horizontally by the chassis  11  at the side surfaces thereof whereat the two end plates  64 ,  65  are vertically swung by the hydraulic pressure generated by the hydraulic cylinders  88 ,  89 . A fixed axle plate  86  is connected to the base portion of the hydraulic cylinder  88  by a pin and it has a configuration of a notchlike claw of a crab that is opened at right angles as viewed from the side surface and it is fixed to the chassis  11 . Likewise, a fixed axle plate  87  is connected to the base portion of the hydraulic cylinder  89  by a pin and it has a configuration of a notchlike claw of a crab that is opened at right angles as viewed from the side surface, and it is fixed to the chassis  11 . A cylinder rod  90  is telescopically inserted into the tip end of the hydraulic cylinder  88 , and a fixed metal fitting attached to the tip end of the cylinder rod  90  can engage with the upper end of the end plate  64  at the long side. When a pin  92  is inserted through the fixed metal fitting of the cylinder rod  90  and the pin hole  78  of the end plate  64 , the cylinder rod  90  and end plate  64  are pivotally connected to each other. Likewise, a cylinder rod  91  is telescopically inserted into the tip end of the hydraulic cylinder  89 , and a fixed metal fitting attached to the tip end of the cylinder rod  91  can engage with the upper end of the end plate  65  at the long side. When a pin  93  is inserted through the fixed metal fitting of the cylinder rod  91  and the pin hole  79  of the end plate  65 , the cylinder rod  91  and end plate  65  are pivotally connected to each other. 
     The axle portion  62  is connected to the lower portion of the swing portion  61  and has the rear iron wheels  25 ,  26  respectively supported at the left and right sides thereof. A main constituent of the axle portion  62  is a long rectangular parallelepiped axle supporting portion  96  that is square in cross section, and the length of the axle supporting portion  96  is substantially the same as the length of the connecting body  66 . The central side surface of the axle supporting portion  96  is perforated to form a pin hole  97  in the horizontal direction, and the axle supporting portion  96  is disposed between the pair of axle supporting plates  74 ,  74 . A pin  103  is inserted into the rear axle supporting plates  74 ,  74  and the pin hole  97  so that the axle supporting portion  96  is laterally swingably held between the pair of axle supporting plates  74 ,  74 . The hydraulic motor  98  is fixed to one end of the axle supporting portion  96  and a driving shaft  100  protrudes from the hydraulic motor  98 . The rear iron wheel  25  is fixed to the driving shaft  100 . The hydraulic motor  99  is fixed to the other end of the axle supporting portion  96  and a driving shaft  101  protrudes from the hydraulic motor  99 . The rear iron wheel  26  is fixed to the driving shaft  101 . The driving shafts  100 ,  101  are arranged in the manner that their axle lines are aligned. With such a construction, the axle portion  62  is laterally swingably relative to the swing portion  61  so as to swing the rear iron wheels  25 ,  26  that are supported by the axle portion  62  at the left and right sides thereof while keeping the axial lines between the rear iron wheels  25 ,  26  aligned. Accordingly, even if there occurs a distortion or variation in heights between the rails  28 ,  29  or a distortion in the chassis  11  of the track traveling vehicle  10 , it is possible to always maintain the rear iron wheels  25 ,  26  in contact with the rails  28 ,  29 . 
     FIG. 7 is a side view showing a state where the driving mechanism  24  is assembled as a unit by each component illustrated in the exploded view of FIG.  6  and is fixed to the rear portion of the chassis  11 . Lateral channels  105 ,  106  each having a square pipe shape are fixed to the chassis  11  at the side surface thereof and positioned in front of and behind the left rear tire  14  in the lateral direction so as to protrude therefrom. The notched portion of the fixed axle plate  86  is brought into intimate contact with the upper and rear side surfaces of the lateral channel  105  while the notched portion of the fixed axle plate  81  is brought into intimate contact with the front side and lower surface of the lateral channel  106 . When the fixed axle plate  86  is fixed to the lateral channel  105 , the base portion of the hydraulic cylinder  88  is connected to the chassis  11 . When the fixed axle plate  81  is fixed to the lateral channel  106 , the fixed axle plate  81  is disposed aslant in the direction of the left rear tire  14 , and hence the end plate  64  connected to the fixed axle plate  81  by the pin  83  is turnably suspended between the lateral channel  106  and left rear tire  14 . When hydraulic oil is supplied to the hydraulic cylinder  88 , the cylinder rod  90  telescopes, and the operation force of the cylinder rod  90  is transmitted to the end plate  64  via the pin  92  so that the end plate  64  is turned about the pin  83 . 
     When hydraulic oil is supplied to the hydraulic cylinder  88 , the cylinder rod  90  is extended in the direction A in FIG. 7, so that the motion of the cylinder rod  90  in the direction A is transmitted to the end plate  64  via the pin  83 , thereby tuning the end plate  64  in the direction B in FIG.  7 . Since the common axle  68  is supported by the end plate  64  and the friction wheel  72  is fixed to the common axle  68 , the outer periphery of the friction wheel  72  is brought into intimate contact with left rear tire  14  when the end plate  64  is turned in the direction B. Although the state where the friction wheel  72  is brought into intimate contact with the left rear tire  14  is illustrated in FIG. 7, this state is also illustrated in FIGS. 2 and 3. When the end plate  64  is turned about the pin  83  in the direction B, the connecting body  66  is also swung downward so that the axle supporting portion  96  moves downward. In consequence, the rear iron wheel  25  supported by the axle supporting portion  96  is pressed downward from the position where it is forced to float in the air, then it is brought into intimate contact with the rail  28 . When the rear iron wheel  25  is pressed downward, the left rear tire  14  is lifted from the ground and is suspended in a state where it is forced to float in the air. In these steps, the track traveling vehicle  10  is changed from a state it travels on a public road to a state where it moves on the rail  28 . 
     FIG. 8 shows a more detailed construction of a hydraulic circuit of the hydraulic system in the driving mechanism of track traveling vehicle  10  according to the first embodiment of the invention. When the track traveling vehicle  10  travels on rails  28 ,  29 , the friction wheel  72  is brought into intimate contact with the left rear tire  14  while the friction wheel  73  is brought into intimate contact with the right rear tire  15 . When the left and right rear tires  14 ,  15  are turned, the friction wheels  72 ,  73  are also turned, and hence the turning effort of the friction wheels  72 ,  73  is transmitted to the common axle  68 . Since gear boxes  111 ,  112 ,  113  each having gears inside thereof are connected to both ends and center of the common axle  68 , wherein the turning effort that is reduced by the gears of the gear box  111  is transmitted to the hydraulic pump  69 , while the turning effort that is reduced by the gears of the gear box  112  is transmitted to friction hydraulic pump  70  and the turning effort that is reduced by the gears of the gear box  113  is transmitted to the hydraulic pump  71 . When the common axle  68  is turned, the hydraulic pumps  69 ,  70 ,  71  discharge hydraulic oil at the same time. A gear box  114  having gears therein is connected to an output of the hydraulic motor  98  and the rear iron wheel  25  is connected to an output of the gear box  114 . When the hydraulic oil is supplied to the hydraulic motor  98 , the turning effort outputted by the hydraulic motor  98  drives the rear iron wheel  25  via the gear box  114 . A gear box  115  having gears therein is connected to an output of the hydraulic motor  99  and the rear iron wheel  26  is connected to an output of the gear box  115 . When hydraulic oil is supplied to the hydraulic motor  99 , the turning effort outputted by the hydraulic motor  99  drives the rear iron wheel  26  via the gear box  115 . 
     Both hydraulic output ends of the hydraulic pump  69  are connected to hydraulic input ends of the hydraulic motor  98 , and hence closed circulation circuits are formed by the hydraulic pump  69  and hydraulic motor  98 . Accordingly, the output of hydraulic oil from the hydraulic pump  69  is directly transmitted to the hydraulic motor  98  so that the hydraulic motor  98  is always driven while interlocked with the motion of the hydraulic pump  69 . Likewise, both hydraulic output ends of the hydraulic pump  71  are connected to hydraulic input ends of the hydraulic motor  99 , and hence closed circulation circuits are formed by the hydraulic pump  71  and hydraulic motor  99 . Accordingly, the output of hydraulic oil from the hydraulic pump  71  is directly transmitted to the hydraulic motor  99  so that the hydraulic motor  99  is always driven while interlocked with the motion of the hydraulic pump  71 . 
     A safety circuit  117  is interposed in the circulation circuits of the hydraulic pump  69  and hydraulic motor  98  for preventing hydraulic pressure in the closed circulation circuits from extraordinarily increasing. The safety circuit  117  comprises relief valves  121 ,  122 ,  124  and a flushing valve  123 . The relief valves  121 ,  122  are interposed in a pair of oil paths formed by connecting both hydraulic output ends of the hydraulic pump  69  and hydraulic motor  98  in the manner that the operating directions of the relief valves  121 ,  122  are opposite one another and the neutral position of the flushing valve  123  is connected to the pair of oil paths. Both hydraulic output ends of the hydraulic pump  69  are connected to control ports provided at both ends of the flushing valve  123 . The relief valve  124  is connected to one output port of the flushing valve  123  at the neutral position thereof, and an oil tank  131 , described later, communicates with the relief valve  124 . 
     A safety circuit  118  is interposed in the circulation circuits formed by the hydraulic pump  71  and hydraulic motor  99  for preventing hydraulic pressure in the closed circulation circuits from extraordinarily increasing. The safety circuit  118  comprises relief valve  126 ,  127 ,  129  and a flushing valve  128 . The relief valves  126 ,  127  are interposed in a pair of oil paths formed by connecting both hydraulic output ends of the hydraulic pump  71  and hydraulic motor  99  in the manner that the operating directions of the relief valves  126 ,  127  are opposite one another, and the neutral position of the flushing valve  128  is connected to the pair of oil paths. Both hydraulic output ends of the hydraulic pump  71  are connected to control ports provided at both ends of the flushing valve  128 . The relief valve  129  is connected to one output port of the flushing valve  128  at the neutral position thereof, and the oil tank  131 , described later, communicates with relief valve  129 . 
     The oil supply circuit  119  is formed separately from the pair of driving closed circulation circuits of the hydraulic pump  69  and hydraulic pump  71 . The oil supply circuit  119  comprises the hydraulic pump  70  having a function to supply hydraulic oil to the pair of circulation circuits so as to prevent each circulation circuit from being short of hydraulic oil. Check valves  132 ,  133  which are respectively directed in the forward direction are connected to both hydraulic output ends of the hydraulic pump  70 . The check valves  132 ,  133  communicate with the oil tank  131 . Relief valves  136 ,  137  are connected to both hydraulic output ends of the hydraulic pump  70  so as to be in parallel with the check valves  132 ,  133 , and they communicate with the oil tank  131 . One end of the hydraulic output ends of the hydraulic pump  70  is connected to one end of a hydraulic output ends of the hydraulic pump  69  via the check valve  134  that is directed in the forward direction while one end of the hydraulic output ends of the hydraulic pump  70  is also connected to one end of the hydraulic output ends of the hydraulic pump  71  via the check valve  138  that is directed in the forward direction. The other end of the hydraulic output ends of the hydraulic pump  70  is connected to the other end of the hydraulic output ends of the hydraulic pump  69  via the check valve  135  that is directed in the forward direction while the other end of the hydraulic output ends of the hydraulic pump  70  is also connected to the other end of the hydraulic output ends of the hydraulic pump  71  via the check valve  139  that is directed in the forward direction. 
     The operation of the driving mechanism of a track traveling vehicle according to the first embodiment of the invention is briefly described hereinafter. 
     When the track traveling vehicle  10  is forced to travel on a general public road, hydraulic oil is supplied to each hydraulic cylinder  47  so that the cylinder rod  48  is pulled into the hydraulic cylinder  47 . Then the cylinder rod  48  is pulled upward so that the inner tube  44  connected to the cylinder rod  48  is accommodated into the outer tube  43  and contracted. As a result, the front iron wheels  22  are lifted upward from the rails  28 ,  29  and the front tires  12  contact the ground. Further, hydraulic pressure is applied to the hydraulic cylinders  88 ,  89  so that the cylinder rods  90 ,  91  are pulled into the hydraulic cylinders  88 ,  89 . The end plates  64 ,  65  are turned about the pins  83 ,  84  so as to be lifted upward via the pins  92 ,  93  respectively connected to the tip ends of the cylinder rods  90 ,  91 . As the result, the rear iron wheels  25 ,  26  are lifted upward away from the ground to a height so that the rear tires  13 ,  14 ,  15 ,  16  contact the ground. In such a manner, the track traveling vehicle  10  can travel on a public road when the rear tires  13 ,  14 ,  15 ,  16  are driven by the vehicle engine. 
     Subsequently, when the track traveling vehicle  10  is forced to travel on the track, hydraulic oil is supplied to each hydraulic cylinder  47  to extend the cylinder rod  48  so that the inner tube  44  is pressed downward from the outer tube  43 . Since the front iron wheel  22  supported by the lower portion of the inner tube  44  is also pressed downward, the front iron wheels  22  contact the rails  28 ,  29  to lift the chassis  11 . At the same time, the front tires  29  are lifted upward from the ground, so that the front side of the track traveling vehicle  10  is supported by the front iron wheels  22 . Further, when hydraulic oil is supplied to the hydraulic cylinders  88 ,  89 , the cylinder rods  90 ,  91  are extended in the direction A in FIG.  7 . Accordingly, the end plates  64 ,  65  are turned about the pins  83 ,  84  in the direction B in FIG. 7 so that the axle supporting portion  96  is pressed downward while the rear iron wheels  25 ,  26  that have been in a raised position are pressed downward. The rear iron wheel  25  contacts the rail  28  and the rear iron wheel  26  contacts the rail  29  to lift the chassis  11  so that the rear tires  13 ,  14 ,  15 ,  16  are forced to float in the air, and the rear portion of the chassis  11  is supported by the rear iron wheels  25 ,  26 . At the same time, as the end plates  64 ,  65  are turned in the direction B in FIG. 7, the friction wheel  72  is brought into intimate contact with the left rear tire  14 . When the friction wheel  73  is brought into intimate contact with the left rear tire  15 , a tuning effort of the left rear tire  15  is transmitted to the friction wheel  73 . 
     Inasmuch as the front iron wheels  22  and rear iron wheels  25 ,  26  contact the rails  28 ,  29  as set forth above, the front tires  12  and the rear tires  13 ,  14 ,  15 ,  16  are forced to float in the air, and the track traveling vehicle  10  travels on the track in this state. 
     The traveling function is described next. Since the friction wheel  72  contacts the left rear tire  14  and the friction wheel  73  contacts the right rear tire  15 , the rear tires  14 ,  15  are turned when an engine in the track traveling vehicle  10  is driven, and the turning effort of the rear tires  14 ,  15  drives the friction wheels  72 ,  73  so as to turn the common axle  68 . The tuning effort of the common axle  68  drives the hydraulic pump  69  via the gear box  111 , the hydraulic pump  71  via the gear box  113 , and the hydraulic pump  70  via the gear box  112 . The hydraulic pumps  69 ,  70 ,  71  discharge hydraulic oil and circulate the hydraulic oil in their respective hydraulic circuits. 
     The hydraulic oil discharged from the hydraulic pump  69  is supplied to the hydraulic motor  98  so that the hydraulic motor  98  is driven to output a turning effort. This turning effort of the hydraulic motor  98  turns the rear iron wheel  25  via the gear box  114 . The hydraulic oil discharged from the hydraulic pump  71  is supplied to the hydraulic motor hydraulic motor  99  so that the hydraulic motor  99  is driven to output a turning effort. This turning effort turns the rear iron wheel  26  via the gear box  115 . When these rear iron wheels  25 ,  26  are turned, the track traveling vehicle  10  can move on the rails  28 ,  29 . If the pressure in the closed circuits formed by the hydraulic pump  69  and hydraulic motor  98  becomes extraordinarily high when the hydraulic oil is circulated, either the relief valve  121  or relief valve  122  is opened to allow the hydraulic oil to flow through the other oil path to prevent the pressure from extraordinarily increasing. Further, if the amount of the hydraulic oil increases in the closed circuits, the hydraulic oil is supplied to a pilot side of the flushing valve  123  to switch the flushing valve  123  so that the hydraulic oil is supplied in the direction of the relief valve  124 , and the surplus hydraulic oil is returned to the oil tank  131 . Likewise, even if the pressure in the closed circuits formed by the hydraulic pump  71  and hydraulic motor  99  becomes extraordinarily high, either a relief valve  126  or relief valve  127  is opened to allow the hydraulic oil to flow through the other oil path to prevent the pressure from extraordinarily increasing. Further, if the amount of the hydraulic oil increases in the closed circuits, the hydraulic oil is supplied to a pilot side of flushing valve  128 , to switch the flushing valve  128  so that the hydraulic oil is supplied in the direction of relief valve  129 , and the surplus hydraulic oil is returned to the oil tank  131 . 
     Although the hydraulic pump  70  sucks the hydraulic oil from the oil tank  131  and discharges it when the common axle  68  is turned, the hydraulic oil is supplied to the closed circuits formed respectively by the hydraulic pump  69  and hydraulic pump  71 . When the hydraulic pump  70  is driven in one direction, the hydraulic pump  70  sucks the hydraulic oil from the oil tank  131  via the check valve  132 , and discharges the hydraulic oil to the discharge side of the hydraulic pump  69  via the check valve  134  and also supplied to the discharge side of the hydraulic pump  71  via the check valve  138 . If a pressure of the hydraulic oil discharged by the hydraulic pump  70  becomes extraordinarily high, the relief valve  137  is opened to return the hydraulic oil to the oil tank  131 . On the other hand, when the common axle  68  is reversely returned, the direction of the hydraulic oil discharged by the hydraulic pump  70  is opposite to the forgoing direction. At this time, the hydraulic pump  70  sucks the hydraulic oil from the oil tank  131  via the check valve  133 , and discharges the hydraulic oil to the discharge side of the hydraulic pump  69  (hydraulic output end opposite to the forgoing hydraulic output end) via the check valve  135 , and also supplied to the discharge side (hydraulic output end opposite to the forgoing hydraulic output end) via the check valve  139 . If the pressure of the hydraulic oil discharged by the hydraulic pump  70  becomes extraordinarily high, the relief valve  136  is opened to return the hydraulic oil to the oil tank  131 . The oil supply circuit  119  operates, then the hydraulic pumps  69 ,  71  are operated so that the hydraulic oil from the hydraulic pump  70  is supplied to the closed circuits formed by the hydraulic pumps  69 ,  71  for preventing the closed circuits from being short of hydraulic oil, thereby always automatically supplying an appropriate amount of hydraulic oil to be circulated in the closed circuits. 
     SECOND EMBODIMENT 
     FIGS. 9 to  13   
     A driving mechanism of a track traveling vehicle according to a second embodiment of the invention is described now with reference to FIGS. 9 to  13 . 
     Components of the driving mechanism of a track traveling vehicle of the second embodiment which are common to those of the first embodiment are identified by the same reference numerals and the explanation thereof is omitted. In the second embodiment, different from the first embodiment, hydraulic oil is discharged by the turning force of rear tires  14 ,  15  of the track traveling vehicle  10 , and the thus discharged hydraulic oil drives front iron wheels  22  for allowing the track traveling vehicle  10  to travel on the track. Accordingly, the track traveling vehicle  10  employs the same construction as a general vehicle, namely a front wheel drive vehicle. The reason why such a construction is employed is that there is a case where a load is applied to the front iron wheels  22  depending on equipment to be placed or loaded on the track traveling vehicle  10 , and in such a case, it is very efficient to drive the entire track traveling vehicle  10  by the front iron wheels  22 . 
     FIG. 9 shows a mechanism for generating a hydraulic pressure according to the second embodiment of the invention. FIG. 9 corresponds to FIG. 6, wherein components in FIG. 9 that are common to those in FIG. 6 are depicted by the same reference numerals. The forgoing end plates  64 ,  65  have respectively an L-shape, and bearings  161 ,  162  are respectively fixed to the corner portions of the L-shaped end plates  64 ,  65 . A long common axle  68  is turnably supported by the bearings  161 ,  162  and both ends of the common axle  68  protrude to the left and right from the side surfaces of the end plates  64 ,  65 . A friction wheel  72  is fixed to one end (left front side in FIG. 9) of the common axle  68  while a friction wheel  73  is fixed to the other end (right outermost side in FIG. 9) of the common axle  68 . A long connecting body  66  is extended between the end plates  64 ,  65  at the lower sides thereof, thereby forming a swingable U-shaped frame or skeleton. A rectangular parallelepiped gear box  165  is fixed to the upper surface of the connection body  66  at the center thereof, and the entire gear box  165  is disposed to be directed in the front of the chassis  11  while the common axle  68  is turnably inserted into the side surface of the gear box  165  at substantially a central portion thereof. 
     The gear box  165  comprises a rectangular parallelepiped body that is hollow inside thereof and is formed of and surrounded by a thin steel plate at the periphery thereof, and a part of the gear box  165  at the lower surface is placed on and fixed to the upper surface of the connecting body  66 . A plurality of gears, described later, are mounted in the gear box  165 , wherein the driving force from the common axle  68  is transmitted to hydraulic pumps  166 ,  167 ,  168  when it is transmitted by the respective gears. The hydraulic pump  166  is fixed to one side surface (at the side of the end plate  65 ) of the gear box  165 , and the hydraulic pumps  167 ,  168  are fixed to the other side surface (at the side of the end plate  64 ) of the gear box  165 . When these hydraulic pumps  166 ,  167 ,  168  are operated, the friction wheels  72 ,  73 , and the common axle  68  are respectively turned by the turning effort of rear tires  14 ,  15  so that the turning effort of the rear tires  14 ,  15  is converted into a force for discharging hydraulic oil. 
     A rectangular parallelepiped axle supporting portion  96  is laterally swingably connected to the lower surface of the connecting body  66 , and driving shafts  100 ,  101  protrude respectively from left and right side surfaces of the axle supporting portion  96 . The rear iron wheel  25  is turnably supported by the driving shaft  100  while the rear iron wheel  26  is turnably supported by the driving shaft  101  so that the rear iron wheels  25 ,  26  are held by the driving shafts  100 ,  101  so as to idle. Accordingly, the rear iron wheel  25 ,  26  contact rails  28 ,  29 , so that the chassis  11  travels on the rails  28 ,  29  but they do not drive the chassis  11 . 
     FIG. 10 shows an internal construction of the gear box  165 . The gear box  165  is assembled by a thin steel plate at the upper and lower portions and a periphery thereof and is hermetically closed. In FIG. 10, only side plates  170 ,  171  respectively positioned at the left and right sides are illustrated, while side plates at the upper and lower sides, and side plates at the front and rear sides are removed. The side plate  170  is formed of a rectangular thin steel plate which stands upright, and one end thereof is bent outward at right angles to form a leg portion, and the leg portion is fixed to the upper surface of the connecting body  66 . The side plate  170  is perforated to form an axle hole  172  at a substantially central portion thereof, a through hole  173  at the innermost side (left innermost side in FIG. 10) and another through hole  174  at the front side thereof. The side plate  171  is formed of a rectangular thin steel plate which stands upright, and one end thereof is bent outward at right angles to form a leg portion, and the leg portion is fixed to the upper surface of the connecting body  66 . The side plate  171  is perforated to form an axle hole  175  at a substantially central portion thereof, and a through hole  176  at the front side thereof at a slightly innermost side. 
     The two side plates  170 ,  171  are arranged in parallel with each other while they are spaced apart, and a large gear  181 , a middle gear  182  and small gears  183 ,  184  are respectively inserted between the side plates  170 ,  171 . These gears mesh with one another, and they are arranged in a row in the order of the small gear  184 , large gear  181 , middle gear  182 , small gear  183  in the direction from the right front side to the left innermost side in FIG.  10 . The common axle  68  penetrates the axle hole  172 , large gear  181  and axle hole  175  and it is turnably supported by the axle holes  172 ,  175 , and the common axle  68  and the large gear  181  are connected to each other to be nonrotatably connected with each other. The hydraulic pump  166  is fixed to the side plate  171  at the outside thereof, and a pump shaft  185  protruded from the side surface of the hydraulic pump  166  is inserted into the through hole  176 , and the middle gear  182  is fixed to the shaft  185 . The hydraulic pump  167  is fixed to the side plate  170  at the outside and a pump shaft  186  protruded from the side surface of the hydraulic pump  167  is inserted into the through hole  173 , and the small gear  183  is fixed to the shaft  186 . Further, the hydraulic pump  168  is fixed to the side plate  170  at the outside thereof and a pump shaft  187  protruded from the side surface of the hydraulic pump  168  is inserted into the through hole  174 , and the small gear  184  is fixed to the shaft  187 . If the gear box  165  is assembled in such a manner, the large gear  181 , middle gear  182 , small gears  183 ,  184  are respectively turned while they are interlocked (i.e., meshed) with one another. Accordingly, if the common axle  68  is turned, the shafts  185 ,  186 ,  187  are driven at the same time via these gears. 
     FIGS. 11 and 12 show the construction of each front jack mechanism  21  according to the second embodiment of the invention. Although the front jack mechanism  21  has the same mechanism as illustrated in FIGS. 4 and 5 for telescoping the inner tube  44 , it has a mechanism built in the lower portion of the inner tube  44 , different from the first embodiment, for driving the front iron wheel  22 . 
     A square closing plate  49  is horizontally fixed to the lower end of the inner tube  44 , and upper sides of flat plate shaped axle supporting plates  191 ,  192  are fixed to the lower surface of the closing plate  49  at the left and right ends (right front side and left innermost side in FIG.  11 ). Both axle supporting plates  191 ,  192  have a trapezoidal shape which is widened toward the lower direction as viewed from the side surfaces thereof, and they are arranged in parallel with each other while they are spaced, and they are assembled so as to form a U shape that is opened downward. As shown in FIG. 12, the side surface of the axle supporting plate  191  is perforated to form an axle hole  193  and the side surface of the axle supporting plate  192  is perforated to form an axle hole  194 . The front iron wheel  22  is inserted into the space between the axle supporting plates  191 ,  192 , and an axle shaft  52  is inserted into the axle hole  194 , the central axis of the front iron wheel  22  and the axle hole  193  in this order, thereafter the front iron wheel  22  and the axle shaft  52  are fixed to each other. As a result, the front iron wheel  22  and the axle shaft  52  are integrated with each other so that the axle shaft  52  is turnably supported by the axle holes  193 ,  194 . 
     A motor attachment plate  195  is fixed to the rear side portions of the axle supporting plates  191 ,  192 . The motor attachment plate  195  is formed of a thin steel plate which is belt in an L shape, and it is fixed to the rear side portions of the axle supporting plates  191 ,  192  at both ends on flat surfaces thereof by welding or the like. The L-shaped bent portion of the motor attachment plate  195  is arranged in parallel with the flat surface of the axle supporting plate  191 . A hydraulic motor  196  is fixed to the motor attachment plate  195 , and a driving shaft  197  of the hydraulic motor  196  protrudes outward from the L-shaped bent flat surface of the motor attachment plate  195 , wherein an axial line of the driving shaft  197  is arranged in parallel with that of the axle shaft  52 . A large sprocket  199  is fixed to one end of the axle shaft  52  protruding from the axle supporting plate  191  and a small sprocket  198  is fixed to the driving shaft  197 . An endless chain  200  is wound between the small sprocket  198  and large sprocket  199 . With this construction, when the hydraulic motor  196  is driven, the driving shaft  197  is turned, and the turning effort of the driving shaft  197  is transmitted to the front iron wheel  22  via the small sprocket  198 , endless chain  200 , large sprocket  199  and axle shaft  52 . 
     FIG. 13 shows a construction of a hydraulic circuit of the hydraulic system in the driving mechanism of the track traveling vehicle  10  according to the second embodiment of the invention. 
     Both hydraulic input ends of the hydraulic motor  196  are connected to both hydraulic output ends of the hydraulic pump  166 , wherein closed circulation circuits are formed by the hydraulic pump  166  and the hydraulic motor  196 . A hydraulic oil discharged from the hydraulic pump  166  directly flows to the hydraulic motor  196 , and the hydraulic motor  196  is operated while interlocked with the operation of the hydraulic pump  166 . Likewise, both hydraulic input ends of a hydraulic motor  196 - 2  (although not shown in FIGS. 1 and 2, the same mechanism as the front jack mechanism  21  shown in FIGS. 11 and 12 is disposed on the opposite side of the track traveling vehicle  10 . Although the other front jack mechanism  21  is not shown in FIGS. 11 and 12, there is provided the hydraulic motor  196 - 2  corresponding to the hydraulic motor  196 , front iron wheel  22 - 2  corresponding to the front iron wheel  22 , endless chain  200 - 2  corresponding to the endless chain  200 ) are connected to both hydraulic output ends of the hydraulic pump  167 , wherein closed circulation circuits are formed by the hydraulic pump  167  and the hydraulic motor  196 - 2 . A hydraulic oil discharged from the hydraulic pump  167  directly flows to the hydraulic motor  196 - 2 , and the hydraulic motor  196 - 2  is operated while interlocked with the operation of the hydraulic pump  167 . 
     A safety unit is built in the hydraulic circuit shown in FIG. 13 for preventing hydraulic equipment from being destroyed when hydraulic pressure inside the closed circuits becomes extraordinarily. First, a safety circuit  206  is interposed between the circulation circuits formed by the hydraulic pump  166  and hydraulic motor  196  while a safety circuit  207  is interposed between the circulation circuits formed by the hydraulic pump  167  and hydraulic motor hydraulic motor  196 - 2 . The safety circuit  206  comprises relief valves  211 ,  212 ,  214  and a flushing valve  213 . The relief valves  211 ,  212  are respectively interposed in a pair of oil paths connecting between both hydraulic output ends of the hydraulic pump  166  and hydraulic motor  196  so as to be opposite one another in operating direction, and they are connected to the neutral side of the flushing valve  213 . Both hydraulic output ends of the hydraulic pump  166  are connected to control ports provided at both ends of the flushing valve  213 . The relief valve  214  is connected to one of output ports of the flushing valve  213  at the neutral side, and it communicates with an oil tank  231 . 
     The safety circuit  207  comprises relief valves  221 ,  222 ,  224  and a flushing valve  223 . The relief valves  221 ,  222  are respectively interposed in a pair of oil paths connecting between both hydraulic output ends of the hydraulic pump  167  and hydraulic motor  196 - 2  so as to be opposite one another in operating directions, and they are connected to the neutral side of the flushing valve  223 . Both hydraulic output ends of the hydraulic pump  167  are connected to control ports provided at both ends of the flushing valve  223 . The relief valve  224  is connected to one of output ports of the flushing valve  223  at the neutral side, and it communicates with the oil tank  231 . 
     An oil supply circuit  208  is formed separately from the pair of circulation circuits formed by the closed hydraulic pumps  166 ,  167 . The oil supply circuit  208  is formed of the hydraulic pump  168  and can always supply hydraulic oil to the pair of circulation circuits, thereby preventing the circulation circuits from being short of hydraulic oil. A pair of check valves  215 ,  216  that are directed in backward direction are serially connected to both hydraulic output ends of the hydraulic pump  166  while a pair of check valves  225 ,  226  that are directed in backward direction are serially connected to both hydraulic output ends of the hydraulic pump  167 . Each one end of check valves  232 ,  233  that are directed in forward direction is connected to both hydraulic output ends of the hydraulic pump hydraulic pump  168  while each one end of the check valves  232 ,  233  communicates with the oil tank  231 . Each one end of check valves  234 ,  235  that are directed in forward direction is connected to both hydraulic output ends of the hydraulic pump  168  while each other end of the check valves  234 ,  235  is connected between the check valves  215 ,  216  and the check valves  225 ,  226 . A relief valve  236  is connected to each other end of the check valves  234 ,  235  while a free or terminal end of the relief valve  236  is returned to the oil tank  231 . 
     The operation of the driving mechanism of the track traveling vehicle according to the second embodiment of the invention is briefly described hereinafter. 
     When the track traveling vehicle  10  is forced to travel on the rails  28 ,  29 , the hydraulic cylinders  88 ,  89  are operated to extend the cylinder rods  90 ,  91 . Then, the end plates  64 ,  65  are turned clockwise about the pins  83 ,  84  in FIG. 9, so that the friction wheel  72  contacts the outer periphery of the left rear tire  14  while the friction wheel  73  contacts the outer periphery of the right rear tire  15 . If an engine and a transmission respectively housed inside the track traveling vehicle  10  are operated in this state to drive the rear tires  14 ,  15 , the turning effort thereof turns the friction wheels  72 ,  73 . When the friction wheels  72 ,  73  are turned, the common axle  68  is also turned so that the turning effort of the common axle  68  is converted into a force for discharging hydraulic oil so as to generate a motive power for moving the track traveling vehicle  10  on the rails  28 ,  29   
     When the common axle  68  is turned, the common axle  68  is turned in the axle holes  172 ,  175  to drive the large gear  181  in FIG.  10 . When the large gear  181  is turned, the middle gear  182  and small gear  184  respectively meshing with the large gear  181  are driven to turn the middle gear  182  and small gear  184  in the direction opposite to the large gear  181 . At the same time, since the small gear  183  meshes with the middle gear  182 , it is turned in the same direction as the common axle  68 . When the middle gear  182  and small gears  183 ,  184  are turned, the shafts  185 ,  186 ,  187  are turned so that the respective hydraulic pumps  166 ,  167 , 168  are operated to discharge hydraulic oil. As shown in FIG. 13, since the hydraulic motor  196  is connected to both hydraulic output ends of the hydraulic pump  166 , the discharged hydraulic oil flows in the circulation circuits and is supplied to the hydraulic motor  196  so that the hydraulic motor  196  is driven. When the hydraulic motor  196  is driven, the driving shaft  197  of the hydraulic motor  196  is turned as shown in FIG. 12 so that the turning effort of the driving shaft  197  is transmitted to the small sprocket  198 , endless chain  200 , large sprocket  199  and axle shaft  52  in this order, thereby turning the front iron wheel  22 . Since the front iron wheel  22  contacts the rail  28 , it is turned by the hydraulic motor  196  so that the track traveling vehicle  10  travels on the track. 
     Likewise, when the hydraulic pump  167  is operated to discharge hydraulic oil, the hydraulic oil flows in the closed circulation circuits formed by the hydraulic pump  167  and hydraulic motor  196 - 2  as shown in FIG. 13 so that the hydraulic oil is supplied to the hydraulic motor  196 - 2 . The hydraulic motor  196 - 2  is driven by the hydraulic oil and the turning effort of the hydraulic motor  196 - 2  turns the front iron wheel  22 - 2 . Since the front iron wheels  22 - 2  contacts the rail  29 , the track traveling vehicle  10  travels on the track when the front iron wheel  22 - 2  is turned. 
     If the pressure of the hydraulic oil which flows in the circulation circuits becomes high while the hydraulic pumps  166 ,  167  are operated, the pressure is automatically forced to escape by the safety circuits  206 ,  207 , thereby preventing equipment from being damaged. In the circulation circuits formed by the hydraulic pump  166  and hydraulic motor  196 , if pressure in either flow path of the circulated circuits exceeds a prescribed value, either the relief valve  211  or  212  is operated so as to let hydraulic oil escape from an oil path having a high pressure to an oil path having a low pressure. If the amount of hydraulic oil inside the circulation circuit increases, a pressure is applied from either oil path to a pilot port so that the hydraulic oil is returned from the oil path having a high pressure to the oil tank  231  via the relief valve  214  while the flushing valve  213  is switched. Likewise, in the circulation circuits formed by the hydraulic pump  167  and hydraulic motor  196 - 2 , if a pressure in either circulated circuit exceeds a prescribed value, either the relief valve  221  or relief valve  222  is operated so as to allow hydraulic oil to escape from an oil path having a high pressure to an oil path having a low pressure. If the amount of hydraulic oil inside the circulation circuits increases, a pressure is applied from either oil path to a pilot port so that the hydraulic oil can be returned from the oil path having a high pressure to the oil tank  231  via the relief valve  224  while the flushing valve  223  is switched. 
     In the hydraulic circuit shown in FIG. 13, the oil supply circuit  208  is provided for automatically supplying hydraulic oil to the two circulation circuits so as to prevent each circulation circuit from being short of hydraulic oil. When the hydraulic pump  168  is driven by the turning effort of the common axle  68 , hydraulic oil discharged from the hydraulic pump  168  is always supplied to the two closed circulation circuits. That is, when the hydraulic pump  168  is driven, the hydraulic oil stored in the oil tank  231  passes through either the check valve  232  or check valve  233 , then it is sucked. The hydraulic oil that is discharged from either end of the hydraulic output ends of the hydraulic pump  168  passes through either the check valve  234  or check valve  235  and flows into the two circulation circuits. The hydraulic oil that flows out from either the check valve  234  or check valve  235  passes through either the check valve  215  or check valve  216 , and it is supplied to the suction side of the hydraulic pump  166 . Likewise, the hydraulic oil that flows out from either the check valve  234  or check valve  235  passes through either the check valve  225  or check valve  226 , and it is supplied to the suction side of the hydraulic pump  167 . As a result, the hydraulic oil is always supplied to the circulation circuits formed by the hydraulic pump  166  and the hydraulic motor  196  and the circulation circuits formed by the hydraulic pump  167  and hydraulic motor  196 - 2 . 
     The relief valve  236  is operated to prevent the hydraulic oil supplied from the oil supply circuit  208  from being excessively supplied to the two circulation circuits. As mentioned above, although the hydraulic oil discharged from the hydraulic pump  168  flows out from either the check valve  234  or check valve  235 , the relief valve  236  is released or opened when a pressure of the hydraulic oil at the position where the hydraulic oil flows out becomes high so as to reduce the pressure of the hydraulic oil by returning the hydraulic oil to the oil tank  231 . The construction of the hydraulic circuit according to the second embodiment of the invention shown in FIG. 13 is more simplified compared with that of the first embodiment of the invention so that an appropriate amount of hydraulic oil is always supplied to the hydraulic circuits when the hydraulic pump  168  is operated at the same time with the hydraulic pumps  166 ,  167 . 
     Since the driving mechanism of a track traveling vehicle of the invention has been constructed as set forth above, it is possible to operate the hydraulic pumps by the turning effort of the rubber tires for driving use, and the hydraulic oil discharged from the hydraulic pumps operate the hydraulic motors for turning the iron wheels so that the construction of the driving mechanism of a track traveling vehicle is simplified. It is not necessary to mount hydraulic pumps directly connected to an engine of a vehicle on a chassis, as has been done conventionally in track traveling vehicles, thereby dispensing with a process or step to mount the hydraulic pumps on the chassis so that the driving mechanism of the track traveling vehicle can be easily assembled. Further, with the hydraulic pumps directly connected to an engine, it was necessary to employ expensive variable discharge amount hydraulic pumps because the discharging amount of hydraulic oil must be controlled by stages. According to the invention, it is possible to discharge hydraulic oil by inexpensive fixed discharge amount hydraulic pumps so as to turn the iron wheels, thereby allowing the track traveling vehicle to travel on the track. An rpm of the iron wheels, namely, a velocity of the track traveling vehicle can be controlled by an rpm of the rubber tires so that the velocity of the track traveling vehicle travelling on the track can be controlled in the same operating manner as a conventional truck so that the track traveling vehicle can be easily operated. 
     Since the driving mechanism has a skeleton construction comprising the swing portion swingably fixed to the chassis, and the friction wheels are supported by the swing portion, the friction wheels can be selectively forced to contact or move away from the rubber tires when the swing portion is operated by the hydraulic cylinders. When the friction wheels are brought into contact with the rubber tires, the friction wheels are driven to operate the hydraulic pumps connected to the friction wheels so that the hydraulic oil can be discharged. When the swing portion is turned, the discharge of the hydraulic oil in the track traveling vehicle  10  can be switched, thereby facilitating the operation. Since the driving mechanism can be assembled as a single unit, if the driving mechanism as a unit is connected to the chassis, a vehicle such as a truck can be reformed to form the track traveling vehicle. The reforming of the vehicle can be easily and quickly made by merely attaching a unit to the vehicle without reforming the body of the vehicle to a large extent. Further, since piping through which hydraulic oil flows can be drawn or turned around within the area of the unit, piping need not be drawn or turned around for a long distance extending from the chassis to the hydraulic motors for driving the rear iron wheels. 
     Further, since the axle supporting portion for supporting the rear iron wheels is provided under the swing portion of the driving mechanism in parallel therewith, when the swing portion is driven by the hydraulic cylinders, the rear iron wheels can be pressed downward against the rails. This operation is performed at the same time when the swing portion is turned to allow the friction wheels to contact the rear rubber tires. Accordingly, it is possible to simultaneously perform the function to allow the friction wheels to contact the rear rubber tires and the function to press down the rear iron wheels against the rails so that the rear rubber tires are forced to float in the air, and the travelling on the track and that on a public road can be swithed when the swing portion is turned so that the switching operation can be easily made. 
     The driving mechanism comprises the swing portion and the axle supporting portion, and wherein the axle supporting portion can be swung laterally relative to the swing portion. Since the rear iron wheels are supported by the axle supporting portion at the left and right thereof, the rear iron wheels can be moved vertically when the swing portion swings so that both rear iron wheels are forced to contact the rails even if the rails have a distortion or difference in height. In such a manner, the distortion in the rails or play in the wheel axle can be adjusted so that the rear iron wheels always contact the rails. Accordingly, the turning effort of the rear iron wheels can be accurately transmitted to the rails so that the track traveling vehicle can travels on the rails with assurance. 
     The hydraulic output ends of the hydraulic pumps for driving use and those of the hydraulic motors are connected to each other to form the closed hydraulic circulation circuits. In the friction wheels, the hydraulic pump for oil supply use is operated separately from the hydraulic pumps for driving use, and they are connected to each other in a manner that hydraulic oil discharged from the hydraulic pump for oil supply use is supplied to the closed hydraulic circulation circuits. Even in the closed circulation circuits, if they are used for a long period of time, there is a possibility that hydraulic oil leaks to lower the pressure of the hydraulic oil. When hydraulic oil is supplied from the hydraulic pump for oil supply use to the closed circulation circuits, the closed circulation circuits are adjusted to keep hydraulic oil necessary for the operation. Further, since the hydraulic pump for oil supply use is driven at the same time with the hydraulic pumps for driving, there does not occur a case of a shortage of hydraulic oil. 
     There are two hydraulic pumps for driving use and two hydraulic motors in the driving mechanism and they are combined with each other to form two closed hydraulic circulation circuits. Each hydraulic motor can drive each rear iron wheel independently so as to absorb the difference in turning between the rear iron wheels and transmit a turning force onto the rails with assurance, thereby allowing the track traveling vehicle to travel on the rails. A hydraulic oil discharged from the hydraulic pump for oil supply use is supplied to the circulation circuits to prevent each circulation circuit from being short of hydraulic oil. Accordingly, it is possible to prevent the hydraulic motors for driving the rear iron wheels from idling so that the rear iron wheels can always be driven by the hydraulic oil discharged from the hydraulic pumps for driving use. 
     The hydraulic output ends of the hydraulic pumps for driving use and those of the hydraulic motors are connected to each other to form the closed hydraulic circulation circuits, and a safety circuit is provided in the oil paths between pressure application sides and application collection sides of the hydraulic circulation circuits for allowing hydraulic oil of one hydraulic circulation circuit to flow to the other hydraulic circulation when a hydraulic oil exceeds a prescribed value. Accordingly, if the rear iron wheels are not turned due to some causes or a pressure of a hydraulic oil in the oil paths becomes extraordinary high, the pressure is forced to escape to the other oil path so as to stop the operation of the hydraulic pumps and hydraulic motors to prevent them from burning out. 
     The hydraulic pumps for driving use and the hydraulic pump for oil supply use are driven by the friction wheels at the same time, and they are connected to each other in a manner that hydraulic oil discharged from the hydraulic pump for oil supply use is supplied to the closed hydraulic circulation circuits formed by the hydraulic pumps for driving use and hydraulic pump for oil supply use. Check valves that are directed in a forward direction relative to the hydraulic pump for oil supply use are interposed between both hydraulic output ends of the hydraulic pump for oil supply use, and relief valves are also connected to the same both hydraulic output ends, wherein the check valves and relief valves communicate with an oil tank respectively. Further, the hydraulic output ends of the hydraulic pump for oil supply use and those of the hydraulic pumps for driving use are mutualy connected to each other via the check valves, wherein the check valves are directed toward the hydraulic pumps for driving use. With this construction, hydraulic oil is sucked from the oil tank through the check valves directed in the forward direction and is supplied to the hydraulic pumps for driving use through the check valve directed in the forward direction. Although the hydraulic pump for oil supply use is operated at the same time with the hydraulic pumps for driving use so that the hydraulic pump for oil supply use discharges hydraulic oil in the normal or reverse turning, the hydraulic output end at the discharge side is reversed. However, since the check valves are interposed at the suction side and discharge side of the hydraulic pump for oil supply use, hydraulic oil discharged from the hydraulic pump for oil supply use is always supplied to the discharge sides of the hydraulic pumps for driving use. Accordingly, hydraulic oil discharged from the hydraulic pump for oil supply use is supplied to the oil path at the pressure application side of the hydraulic circulation circuits formed by the hydraulic pumps for driving use and the hydraulic motors irrespective of the normal and reverse tuning of the friction wheels so that the circulation circuits are prevented from being short of hydraulic oil. 
     Although a particular preferred embodiment of the invention has been disclosed in detail for illustrative purposes, it will be recognized that variations or modifications of the disclosed apparatus, including the rearrangement of parts, lie within the scope of the present invention.