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FIELD OF THE INVENTION 
     The present invention concerns a method and system for extending railway track downstream of one end of existing track. 
     BACKGROUND 
     To place in position the cross-ties and rails required for building railway lines, known methods and systems are generally heavy and cumbersome. 
     Various means have already been proposed for streamlining railway track laying systems. 
     Document GB 2432564 describes a machine arranged to lay track panels alongside a wagon transporting track panels, by means of a pivoting arm. The machine described is suitable for creating new track alongside pre-existing track. It appears to be difficult, even impossible, to use the described machine for creating new track if there is no pre-existing track parallel to the new track. 
     Document WO2009/050439 uses means of small size in particular one or more vehicles capable of moving outside the track which do not require the existence of pre-existing adjacent track. However the described means, which only allow rails to be moved, clearly cannot be used if cross-ties are not previously laid. 
     SUMMARY OF THE INVENTION 
     The present invention sets out to overcome the drawbacks of the known prior art. 
     For this purpose, so as to extend a railway track downstream of one end of existing track, the subject of the invention is a method comprising: 
     a step to convey a current track panel whereby an operator causes a motorized lorry to travel on the existing track, the lorry comprising rail wheels and a retracted mechanism for off-track travel and carrying the said current panel, so as to bring the motorized lorry close to the said track end; 
     a step for positioning the current panel downstream of the existing track, whereby the operator deploys the off-track travel mechanism so as to position an upstream end of the current panel directly above the end of the existing track by causing the motorized lorry to travel downstream of the existing track; 
     a step to release the current panel whereby the operator evacuates the motorized lorry leaving the upstream end of the current panel directly above the end of the existing track; and 
     a step to lay the current panel whereby the operator lowers the current panel until the upstream end of the current panel coincides with the end of the existing track. 
     Advantageously, to deploy the off-track travel mechanism: 
     the conveying step comprises a first transient phase in which the operator brings the motorized lorry near, causing it to carry connecting track bracket-suspended at the front, until a rear end of the connecting track is placed directly above the end of the existing track; 
     the positioning step comprises a second transient phase in which the operator lowers the connecting track in the continuation of the existing track, a third transient phase in which the operator causes the motorized lorry to move forward along the connecting track until it is possible to deploy the off-track travel mechanism off the existing track, and a fourth transient phase in which the operator deploys the off-track travel mechanism. 
     In particular to evacuate the motorized lorry, the release step comprises a fifth transient phase in which the operator uses the off-track travel mechanism to return the motorized lorry over the connecting track, a sixth transient phase in which the operator retracts the off-track travel mechanism to re-position the rail wheels of the motorized lorry in contact with the connecting track, a seventh transient phase in which the operator drives the motorized lorry back from the connecting track onto the existing track so as to take hold of the connecting track, an eighth transient phase in which the operator causes the connecting track to be lifted, and a ninth transient phase in which the operator reverses the motorized lorry bringing the connecting track to above the existing track. 
     Advantageously also, to leave the upstream end of the current panel directly above the existing track end: 
     the conveying step comprises a first laying phase in which the operator hooks at least one first extendable gantry crane onto the current panel. 
     the positioning step comprises a second laying step whereby the operator immobilizes the motorized lorry when the entirety of the current panel is downstream of the existing track, and a third laying phase whereby the operator lowers one or more legs of the said at least first extendable gantry crane until they rest on the ground so as to lift the said gantry which then lifts the current panel to above the motorized lorry. 
     In particular, the laying step comprises a fourth laying phase in which the operator lowers the said at least one first extendable gantry crane so as to lay the current panel in the continuation of the existing track then unhooks the said first extendable gantry crane from the current panel, and a fifth laying phase in which the operator lifts the said first extendable gantry crane. 
     Further advantageously the method comprises a transfer step in which the operator loads a following panel on the motorized lorry so as to re-loop the implementation of the method onto the conveying step at which the following panel becomes the current panel. 
     More particularly, the transfer step comprises a first transfer phase in which the operator brings a beam carried by the motorized lorry to under at least one second extendable gantry crane from which the said following panel is suspended, and a second transfer phase in which the operator lowers the said second extendable gantry crane so as to lay the said following panel on the said beam. 
     More particularly the transfer step comprises a third transfer phase in which the operator lifts the said a least one second extendable gantry crane so as to leave the said following panel on the said beam, and a fourth transfer phase in which the operator moves forward the motorized lorry so as to place the beam underneath the said at least one first extendable gantry crane. 
     Preferably at least one of the transfer phases is conducted during the laying step. 
     To extend railway track downstream of one end of existing track, a further subject of the invention is a system comprising: 
     a motorized lorry which comprises rail wheels to travel on the existing track, and a retractable off-track travel mechanism so as to be able to transport a current track panel on the existing track and off the existing track; 
     at least one first extendable gantry crane arranged to hook the current panel thereupon; and 
     connecting track removably bracket-suspended at the front of the motorized lorry. 
     In particular, the system comprises a beam arranged to support the current panel on the said motorized lorry. 
     Advantageously the system comprises at least one second extendable gantry crane arranged to hook the following panel thereupon. 
     Also advantageously, the system comprises a rail lorry to support the beam by accompanying the movements of the motorized lorry, without leaving the track. 
     Also advantageously, the connecting track comprises two rails of length substantially equal to the length of the motorized lorry, held together by cross-ties of length equal to the spacing between the rails. 
    
    
     
       BRIEF DESCRIPTION OF DRAWING FIGURES 
       The invention will be better understood and other objectives, characteristics, details and advantages thereof will become more clearly apparent in the explanatory description below with reference to the appended schematic drawings given solely as examples illustrating one embodiment of the invention and in which: 
         FIGS. 1 and 2  are side views of a system conforming to the invention during two phases of a conveying step; 
         FIGS. 3 to 7  are side views of the system in  FIGS. 1 and 2  during five phases of a positioning step; 
         FIGS. 8 to 11  are side views of the system in  FIGS. 1 and 2  during four phases of a release step; 
         FIGS. 12 to 14  are side views of the system in  FIGS. 1 and 2  during three phases of a laying step and during three phases of a transfer step; 
         FIG. 15  is a side view of the system ready for repeat of the method according to the invention; 
         FIG. 16  shows steps of the method conforming to the invention, 
         FIGS. 17 to 20  are side and front views of a motorized lorry for implementing the invention; 
         FIG. 21  is a side view of the system illustrated in  FIGS. 1 to 15 ; 
         FIGS. 22 and 23  are views of an implementing detail of the invention, 
         FIG. 24  is a front view of the system in  FIG. 21 ; 
         FIGS. 25   a  and  25   b  are front and perspective views of a first state of the system according to the invention; 
         FIGS. 26   a  and  26   b  are front and perspective views of a second state of the system according to the invention; 
         FIGS. 27 to 29  are perspective views of the system in a disassembled state for transport. 
     
    
    
     DETAILED DESCRIPTION 
     To extend railway track downstream of one end of existing track,  FIG. 16  shows a method which particularly comprises a step  101  for conveying a current track panel, a step  103  for positioning the current panel downstream of the existing track, in which an operator deploys an off-track travel mechanism so as to position an upstream end of the current panel directly above the end of the existing track, a step  105  for releasing the current panel in which the operator retracts the off-track travel mechanism and a step  107  for laying the current panel in which the operator lowers the current panel until the upstream end of the current panel coincides with the downstream end of the existing track. 
     In each of preceding  FIGS. 1 to 12 , substantially in the centre and on the right side, one end  36  of already existing railway track  31  is shown. On the left of the end of the existing track a platform  30  is shown on which the method and system explained below allow the rail track to be extended downstream of the end of the existing track (towards the left in the figure). 
     In  FIGS. 13 to 15 , the end  36  has been offset towards the left so as better to explain some steps which take place on the existing track upstream of the track end  36 . 
       FIG. 16  synthesizes and completes the steps of the method explained with reference to  FIGS. 1 to 15 . 
       FIGS. 1 and 2  respectively illustrate a first laying phase  1  and a first transient phase  2  within the conveying step  101  of a current track panel  32  in which an operator causes a motorized lorry  34  to travel on the existing track  31 , the lorry carrying the current panel  32 , so as to bring the motorized lorry  34  near to the end  36  of the track. 
     Solely as a non-limiting illustration, the operator is a human being, a team of human beings or a programmable logic controller. If the operator is a human being, a portable remote control station allows controlling of the functions which are each servo-controlled for implementing the method. The use of a programmable logic controller is possible provided it is equipped with sensors and a programme that are sufficient to make provision for all the situations which may occur. A human being is able to confront unexpected and varied situations whilst controlling the proper conducting of the method without having to invest in a prohibitive number of sensors and complex combined programming. 
       FIG. 21  illustrates a system which allows the method of the invention to be carried out so as to extend the rail track downstream of the end  36  of existing track  31 . The system particularly comprises a motorized lorry  34 , a beam  34  arranged to support the current panel  32  on the motorized lorry  34 , at least one extendable gantry crane  33  arranged to hook the current panel  32  thereupon, and connecting track  38  removably bracket-suspended at the front of the motorized lorry  34 . 
     In one preferred embodiment illustrated in  FIGS. 17 to 20 , the motorized lorry  34  comprises rail wheels  51  for travelling on the existing track  31  and a retractable off-track travel mechanism. The mechanism  50  is a caterpillar mechanism for example which is lowered by arms  52  to below the level of the rail wheels  51 . Other off-track travel mechanisms can be envisaged such as a wheel mechanism for example equipped with all-terrain tyres. Therefore, in retracted position as illustrated in  FIGS. 17 and 18 , the off-track travel mechanism  50  is lifted to place the rail wheels  51  in contact with the rails  53 . In deployed position as illustrated in  FIGS. 19 and 20 , the off-track travel mechanism  50  is lowered to rest upon the platform  30  so as to lift the motorized lorry  34  until its rail wheels  51  are lifted off the rails  53 . The motorization of the lorry  34  is of hydraulic, remote controllable type for example. 
     The purpose of the beam  44  is to maintain the panel  32  flat. In the absence of a beam, the panel which may be 36 meters in length or more for example, even only 24 meters or less, will tend to bend under its own weight to which must be added the weight of the gantries  33  when it is not laid on the ground, on account of the flexibility of the rails having such lengths. The length of the beam is preferably adapted to the length of the panel  32 . A slightly shorter length allows the system to be made more lightweight without causing any observable bending at the ends. 
     The number of extendable gantry cranes  33  is also a function of the length of the panel  32 . In the example illustrated in  FIG. 21 , three extendable gantry cranes  33  are suitable if the length of the panel  32  is 36 meters. 
       FIG. 25   a  and  FIG. 25   b  respectively illustrate a front view and perspective view of one possible embodiment of the extendable gantry crane resting on rails  41 . The extendable gantry crane  33  illustrated here is equipped with wheels  40  enabling it to travel on the rails  41 . The extendable gantry crane  33  comprises a horizontal telescopic lifting beam  45  which comprises an extendable arm  46  either side. A vertical sleeve  49  is fixed to each end of the extendable arm  46 . A leg  39  in the form of a vertical column is slidingly mounted in each sleeve  49 . In  FIG. 24  the arms  46  are illustrated in retracted position. A hydraulic control unit  48  can be remotely driven to extend the two extendable arms  46  so as to deploy the two sleeves  49  on each side of the motorized lorry  34  then to cause each leg  39  to be lowered until the foot bears upon the ground. 
     Remote controlled clamps  47  allows the current panel  32  to be hooked onto the extendable gantry crane  33  so that when the two legs  39  resting on the ground lift the extendable gantry crane  33 , the panel  32  is lifted off the beam  44 . The clamps illustrated in open position in  FIGS. 25   a  and  25   b  are shown in closed position in  FIGS. 26   a  and  26   b.    
     If several extendable gantry cranes  33  are used they are connected by link bars  54  so as to form a rigid structure. The hydraulic units  48  are then synchronized so as to generate uniform deployment movement then uniform lifting and lowering of the extendable gantry cranes  33 . 
     As explained above, the beam  44  is essentially useful when the panel  32  is of long length. A very short panel could perfectly well be supported directly on the motorized lorry  34 . A panel of average length could be supported by a beam carried by the motorized lorry  34  alone. If the panel  32  is of large size as illustrated in  FIG. 21  the system comprises a rail lorry  35  to support the beam  44 , which accompanies the movements of the motorized lorry  34  without leaving the track. 
     As illustrated in  FIG. 24 , the connecting track  38  comprises two rails of length substantially equal to the length of the motorized lorry  34 , held together by cross-ties  57  of length equal to the spacing between the rails. In other words, the cross-ties do not protrude beyond the sides of the rails outside the connecting track  38 . With this arrangement it is possible to avoid any contact of the off-track travel mechanism  50  with the cross-tie heads of the last laid panel or of the existing track before extension. In the embodiment illustrated in  FIG. 21 , the connecting track  38  is hung from the front end of the beam  44 . If no beam is used other assemblies could be envisaged, for example a specific arm mounted on the motorized lorry  34 . 
     The mounting of the connecting track  38  onto the end of the beam  44  is detailed in  FIG. 22 . The front part of the beam  44  comprises an openwork girder  55  supported by a lattice framework  56  to combine lightweight with horizontal rigidity. The lower part of a hook  58  grasps hold of the connecting track  38 . An upper end of a bar  61  pivots on a pin  63  secured to the girder  55 . The hook  58  pivots on a pin  64  at the lower end of the bar  61 . 
     In the raised position of the connecting track  38  illustrated in a side view in  FIG. 23   b  and a front view n  FIG. 23   a , a jack  60  fixed to the girder  55  in withdrawn position pulls on a cable  59  whose opposite end is attached to the top of the hook  58 . Two cheeks  62  allow the connecting track  38  to be held in raised position in the alignment of the track  31 . 
     In the lowered position of the connecting track  38 , which is illustrated in a side view in  FIGS. 22   b  and a front view in  FIG. 22   a , the jack  60  in projecting position releases the traction on the cable  59  so that the connecting track  8  leaves the two cheeks  62  and comes to be aligned with the track  31 . 
     The hook  64  is of known shape to allow firm gripping of the connecting track in raised position and to allow easy release of the connection track in lowered position when the beam  44  moves forward and the connecting track  38  comes to rest on the ground. 
     At the laying phase  1  illustrated in  FIG. 1  and at the transient phase  2  illustrated in  FIG. 2 , the off-track travel mechanism  50  is in retracted position so as to bring the motorized lorry near to the end of the existing track by causing it to travel on the track  31 . 
     At the laying phase  1  of conveying step  101 , the operator hooks at least one extendable gantry crane  33  onto the current panel  32  resting on the beam  44 , in particular by remote controlling the clamps  47  so that they close firmly around the rails of the current panel  32 . The beam  44  loaded with the panel arrives at the laying front. 
     During transient phase  2  of step  101 , the operator brings the motorized lorry  34  near to the connecting track  38  hanging in raised position in front of the beam  44  until the rear end of the connecting track  38  can be placed directly above the end  36  of the existing track  31 . The placing in direct overhead position of the connecting track then validates transition  102  to step  103  during which the current panel  32  is positioned downstream of the existing track  31 . 
       FIGS. 3 ,  4  and  5  respectively illustrate three transient phases  3 ,  4  and  5  within step  103 . At step  103 , the operator deploys the off-track travel mechanism so that it is possible to cause the motorized lorry  34  to travel downstream of the existing track  31  to position the upstream end  37  of the connecting track  38  directly above the end  36  of the existing track  31 . 
     During transient phase  3  of the positioning step  103 , the operator lowers the connecting track  38  into the continuation of the existing track  31  in particular by remote controlling the jack  60 . 
     During transient phase  4  of step  103  the operator moves the motorized lorry  34  forwards on the connecting track  38  until it is possible to deploy the off-track travel mechanism off the existing track  31 . During transient phase  4  illustrated in  FIG. 4 , the absence of any cross-ties extending beyond the connecting track allows the deployment of the off-track travel mechanism when the mechanism is entirely on the connecting track. The caterpillars or all-terrain tyres of the mechanism are not hindered by cross-ties in order to touch the ground. 
     During transient phase  5  of step  103 , the operator deploys the off-track travel mechanism by remote controlling rotation of the arms  52  which allows simultaneous contact of the off-track travel mechanism  50  with the ground and the eased contact of the wheels  51  with the rails of the connecting track whilst maintaining the motorized lorry  34  horizontal. The operator uncouples the driving motor of the rail wheels and couples the driving motor of the off-track travel mechanism  50 . 
       FIGS. 6 and 7  respectively illustrate two laying phases  6  and  7  within positioning step  103 . 
     During laying phase  6 , the operator moves the motorized lorry  34  forward by means of the caterpillars which bear on the ground either side of the rails of the connecting track  38 . For as long as the rail wheels touch the rails  41  of the connecting track they simply follow by free-wheeling as do those of the rail lorry  35 . The operator continues to move the lorry  34  forward which gradually leaves the connecting track without undergoing any misalignment, whether lateral or vertical. The lorry continues to move forward off track until the entirety of the current panel  32  is downstream of the existing track  31  with the rear end of the panel  32  directly above the downstream end of the existing track  31 , when the operator immobilizes the motorized lorry  34 . The overhang length of the panel  32  at the rear of the beam  44  and the overhang length of the beam  44  at the rear of the rail lorry  35  are such that the rail lorry  35  remains on the connecting track  38  when the rear end of the panel  32  lies directly overhead the downstream end of the existing track  31 . 
     During laying phase  7 , the operator synchronously lowers the legs  39  of the extendable gantry cranes  33  until the legs  39  resting on the ground lift up the gantry  33  whose radio-controlled clamps  47  then lift up the current panel  32  from the beam carried by the motorized lorry  34  leaving the upstream or rear end  37  of the current panel  32  directly above the downstream end  36  of the existing track  31 . The placing in overhead position of the panel  32  validates transition  104  to step  105  in which the panel is released. 
       FIGS. 8 to 12  respectively illustrate five transient phases  8  to  12  within the release step  105 . 
     During transient phase  8 , the operator uses the off-track travel mechanism to bring the motorized lorry  34  back over the connecting track  38 . On reversing, the motorized lorry  34  causes the reversing of the rail lorry  35  on the track  31  by pushing on the beam  44 . The rail lorry  35  engaged on the rails, acts naturally on the beam  44  to guide the motorized lorry  34  mechanically towards the connecting track  38 . 
     During transient phase  9  the operator stops the motorized lorry  34  which has arrived on the connecting track  38  then retracts the off-track travel mechanism to replace the rail wheels of the motorized lorry  34  in contact with the connecting track  38 . The operator then performs a reverse operation to that of phase  5  to couple the driving motor of the lorry  34  with the rail wheels. 
     During transient phase  10 , the operator sends the motorized lorry  34  back from the connecting track  38  onto the existing track  31  to fully evacuate the beam  44  from underneath the panel  32 . By moving back the front end of the beam  44  over the connecting track  38  and with the bar  61  lowered, the hook  64  naturally comes to grasp the connecting track  38 . If, during phase  4  or later, the operator had raised the bar  61  to facilitate moving of the beam  44 , the operator orders the jack  60  to lower the hook  64  over the connecting track before sending back the lorry  34 . 
     During transient phase  11 , the operator lifts up the connecting track  38  for example by commanding withdrawal of the jack  60  which then pulls on the cable  59 . 
     During transient phase  12 , the connecting track  38  being lifted above the level of the track  31 , the operator reverses the motorized lorry  34  which brings the connecting track  38  back above the existing track  31 . The track  31  in the centre of  FIG. 12  is cut to illustrate the reversing of the lorry  34  to well upstream of the downstream end of the track  31  which corresponds to a phase of another step of the method optionally performed simultaneously with phase  12  which is explained later on the description. 
     Therefore, the evacuation of the lorry  34  by the operator, at the end of the release step  105  of the current panel  32 , leaves the upstream end  37  in other words the rear end of the current panel  32  hanging from the clamps  47  of the extendable gantry cranes  33  directly above the end  36  of the existing track  31 . The removed status of the connecting track then validates a transition  106  to the actual laying step  107 . 
       FIGS. 13 and 14  respectively illustrate two laying phases  13  and  14  within the laying step  107  of the current panel  32 . 
     During the laying phase  13  illustrated on the left side in  FIG. 13 , the operator lowers the extendable gantry cranes  33  so as to lay the current panel  32  in the continuation of the existing track  31 . When the panel  32  is laid on the ground, the operator controls opening of the clamps  47  so that the current panel  32  is unhooked from the extendable gantry cranes  33 . 
     At the laying phase  14  illustrated on the left side of  FIG. 14 , the operator orders the extendable gantry cranes  33  to be raised. 
     In this manner, the lowering of the current panel  32  on the ground by the operator, at the end of step  105 , causes the upstream end  37  of the current panel  32  to coincide with the end  36  of the existing track  31 . The current panel  32 , laid on the ground, then extends the existing track  31  by a track panel length thereby validating transition  108  towards possible repeat of the method starting from step  101  when needed. 
       FIGS. 12 to 14  on the right side respectively illustrate three transfer phases  16  to  18  within a transfer step  109  during which the operator loads a following panel  42  on the motorized lorry  34 , more particularly on the beam  44  carried by the motorized lorry, subsequent to validation of the transition  106 . 
     During the first transfer phase  16 , the operator brings the beam  44  carried by the motorized lorry  34  to underneath a set of second extendable gantry cranes  43  from which the following panel  42  is hung. 
     At the second transfer phase  17 , the operator lowers the extendable gantry cranes  43  so as to lay the following panel  42  on the beam  44 . 
     At the third transfer phase  18 , the operator lifts up the second extendable gantry cranes  43  to leave the following panel  42  on the beam  44 . 
     Transfer step  109  is completed by a fourth transfer phase  15  illustrated in  FIG. 15 . At transfer phase  15 , the operator moves the motorized lorry  34  forward so as to place the beam  44  supporting the following panel underneath the first set of extendable gantry cranes  33 . Transition  110  is validated at the end of step  109  when the panel  42  is transferred to under the extendable gantry cranes  33 . The validation of transition  110  allows re-looping of the implementation of the method onto conveying step  101  during which the following panel  42  then becomes the current panel  32 . 
     The performing of one or more transfer phases during laying step  107  provides savings in time for extending the track  31 . 
     For as long as track panels are to be laid one after the other to extend the existing track, each validation of the transition  110  activates a feed step  111  for a supply panel of the method. 
     When the following panel  42  is transferred towards the extendable gantry cranes  33 , the extendable gantry cranes  43  are free for hooking a supply panel thereupon which then becomes the new following panel  42 . The supply can be ensured for example by bringing a transport wagon—not illustrated—to underneath the extendable gantry cranes  43 , on which one or more track panels are stacked to supply the system. 
     The supplied panel validates transition  112  to step  109  ready to be activated subsequent to the next validation of transition  106 . 
     The method of the invention has been described with respect to extending track beyond existing track. Evidently the method can also be applied for track renewal, the former track being removed to leave the way free for laying new track which then forms a track which is extended as and when the former track is removed. 
     In one preferred embodiment a length of beam  44  e.g., between 31 meters and 32 meters as a non-limiting illustration, provides support for track panels 36 meters in length. Advantageously, the beam  44  can be separated into three parts  44   a ,  44   b  and  44   c  which can be loaded onto a platform  81  of a road vehicle as illustrated by the packing in  FIG. 27 . Four link bars  54  can be housed in a hollow part of the beam part  44   c  and four link bars  75  used for rigid maintaining of the extendable gantry cranes  43  can be housed in a hollow part of beam part  44   b . The dimensions of the system elements just described are sufficiently small to leave room on platform  81  of standard size for the stowing of six low bearing pads  73  and six high bearing pads  74 . The six low bearing pads  73  are each designed to be assembled at the base of one of the two legs  39  of an extendable gantry crane  33  so as to facilitate the bearing of the three extendable gantry cranes  33  used for laying. The six high bearing pads  74  are provided for each to be assembled at the base of one of the two legs  39  of the extendable gantry crane  43  so as to facilitate the bearing of the three extendable gantry cranes  43  used for transfer. The bearing pads  74  are raised so as to allow the passing underneath the three extendable gantry cranes  43  of a stack of track panels loaded on a transport wagon. 
     With reference to the packing in  FIG. 28 , a second platform  82  of a road vehicle is suitable for transporting the motorized lorry  34 , the connecting track panel  38 , also called provisional track panel, the rail lorry  35  and two extendable gantry cranes in stowed position. 
     With reference to the packing in  FIG. 29  a third platform  83  of a road vehicle is suitable for transporting four extendable gantry cranes in stowed position. 
     For the sizing of the system of the invention, every possible weight reduction is sought to promote saving in energy whilst preserving the mechanical robustness required for implementing the method of the invention. 
     Three trucks therefore allow the transporting of the system of the invention via road to the construction sites thereby providing large flexibility of use. At initial step  100  of the method, the system is brought on site, preferably when the wagons transporting the track panels are already on site to minimize the stay time of the system on the site. At step  100 , the elements of the system are unloaded from the platforms  81 ,  82  and  83  then the system is assembled. At the end of step  100 , the system is then ready to initiate steps  101  and  111  of the method of the invention. 
     It is noted that the method and system just described are modular and can be adapted to different track panel lengths. The description has been given with respect to panels 36 meters in length which require three extendable gantry cranes. For operation with panels 24 meters in length, it is possible to envisage only sets of two extendable gantry cranes and to reduce the length of the beam by placing end to end the front part  44   a  and the rear part  44   c  without assembling the central part  44   b  of the beam  44 . 
     The method of invention allows rail track of excellent quality to be obtained rapidly and at low cost.

Summary:
A method for extending a railway track downstream from an existing track end includes transporting a common track panel; positioning the common track panel downstream from the existing track, deploying an off-track moving mechanism and positioning an upstream end of the common panel vertically, in line with the existing track end; releasing the common track panel, removing the off-track moving mechanism; and installing the common track panel, by lowering the common track panel until the upstream end of the common track panel is aligned with the existing track end.