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BACKGROUND OF THE INVENTION 
     The invention relates to a rail expansion device comprising a stock rail and a tongue rail, which can be shifted longitudinally relative to each other and are in contact under the application of force acting transversely relative to the rail longitudinal direction. 
     Rail expansion devices—also known as expansion joints—are installed for example in the area of bridges in order to allow relative movements between edifice and track. For this purpose, the tongue rail is arranged moveable relative to the stock rail. For this purpose, the tongue rail, for example, can be immobilized between the stock rail and a fixed clamping jaw on the opposite side. As explained in for example DE-A-1 534 052, tongue rail foot and stock rail foot originate from a common baseplate. 
     In a rail expansion device according to WO-A-93/23624 a stock rail is arranged moveable relative to a tongue rail, whereby the tongue rail is machined to match the curvature of the stock rail. 
     An expansion joint for grooved rails according to DE-A-195 25 135 features a section of track welded to a guide plate, whereby the guide plate is arranged on a sliding element displaceable along the longitudinal direction of the expansion joint. 
     In rail expansion devices known in the art, the tongue rails are usually welded to the adjoining control rail. Expensive work has to be performed if replacement of the tongue rail is made necessary by its state of wear. Moreover, not all materials can be welded together, resulting in the disadvantage that hard-to-weld, long-wearing steel types are not used. 
     SUMMARY OF THE INVENTION 
     It is the objective of the present invention to further develop a rail expansion device of the above type in a way that allows quick replacement of the tongue rail and allows materials to be used for the tongue rail that are non-weldable or consist of hard-to-weld, long-wearing steel types. 
     In order to meet this objective, the invention intends that the tongue rail be accommodated detachably in a holding device—some distance away from the region of the tongue rail located adjacent to the stock rail—and that in the holding device the tongue rail be form-fittingly and force-fittingly connected to a control rail, or a section of a control rail, or a section of the holding device merging into the control rail. 
     Different from rail expansions known in the art, the tongue rail is immobilized by means of a separate holding device, in which the tongue rail is connected to the control rail directly or indirectly in a positive or non-positive manner. The holding device itself may merge directly into a section of the control rail, so that the separable connection is established to a section of the holding device and a remaining section of the control rail. 
     Thus, the employed design is similar to that known from switches for grooved rails, as is described in EP-B-0 603 883 or DE-A-101 14 683, for example. 
     While the holding devices known in the art are solely intended for switch structures in the form of switches for grooved rails, the invention proposes a rail expansion device with a tongue rail with a profile that can correspond to that of a Vignoles rail. 
     However, the invention&#39;s teaching is also applicable to expansion joints for tracks of grooved rails. In such a case it is intended that the stock rail be arranged movable relative to the tongue rail, whereby the tongue rail rests upon a support, which in turn acts as the inner guide as a first guide for the stock rail. On the outer side the stock rail is at least in sections in contact to an outer guide as a second guide. In this, the second guide is connected, e.g. welded, to a baseplate, from which originates a side rail, which in turn is connected, in particular also by welding, to the guide preferably by means of a guide rail. To ensure a secure attachment, the guide rail should engage—with its longitudinal edge region on the side facing away from the stock rail—into a geometrically matching longitudinal recess of the side rail. 
     To prevent a horizontal movement of the tongue rail, spacer elements such as strips, which in particular are bolted to the guide rail, may be placed between the lateral face of the tongue rail facing away from the stock rail and the inner face of the side rail. Vertical movement of the tongue rail can be prevented if the tongue rail undercuts the stock rail in the head region. 
     The invention&#39;s teaching allows a simple mechanical structure particularly for expansion devices for grooved rails that due to the exchangeability of the tongue rail is embodied in a very easy-to-service manner. This in particular allows the option of manufacturing the tongue rail from a hardwearing and non-weldable or hard-to-weld material. This ensures a secure guideway for the stock rail, whereby the inner guide serves a double function in that it not only secures the stock rail against horizontal movement but at the same time provides support for the tongue rail. 
     The guide rail can additionally be supported by supports such as plate elements originating from the baseplate, so that bending in the vertical direction can be ruled out even under high load. 
     The holding device should be embodied with a block-like shape and in particular possesses two channel-like recesses extending along the rail&#39;s longitudinal direction, whereby into one recess are introduced the tongue rail with its heel as well as one wedge element, which is used to immobilize the tongue rail, i.e. to form-fittingly and force-fittingly connect the tongue rail to the adjacent stock rail or a section of the latter. The wedge element subsequently is secured in sections by fastening elements, such as bolts, extending within the second channel-like recess. 
     In order to replace the tongue rail it is now only necessary to detach the spacer elements or strips that are preferably bolted to the guide rail. Subsequently the wedge elements are removed so that the tongue rail can be removed and can be replaced by a new one. On the other hand, no work is required on the stock rail or its outer guide. 
     If parts of the holding device for the tongue rail heel are contained in a housing, the corresponding coverings are to be removed in order to be able to loosen and remove the wedge elements and thus the tongue rail. 
     Independent hereof, the tongue rail should consist of highly wear-resistant steel, such as BAINIT, hard manganese steel, tempered rail steel (R 350 HT grade), or tempered close-grained structural steel, e.g. DILIDUR, HARDOX, XAR in grades 400 to 500. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Further details, advantages, and features of the invention are found not only in the claims and the characteristic features contained therein—individually and/or in combination—but also in the following description of a preferred embodiment example illustrated in the figures. 
         FIG. 1  shows a rail expansion device, 
         FIG. 2  shows a view along direction A of  FIG. 1 , 
         FIG. 3  shows a sectional view along the line B-B in  FIG. 1 , 
         FIG. 4  shows a sectional view along the line C-C in  FIG. 1 , and 
         FIG. 5  shows a sectional view along the line D-D in  FIG. 1 . 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The figures purely schematically illustrate a rail expansion device  10  relating to grooved rails, but this is not to be interpreted as a limitation of the teaching according to the invention. On the contrary, the invention&#39;s teaching is also applicable for other rail profiles, in particular Vignoles rail. 
     The rail expansion device  10  in the usual manner comprises stock rails  12 ,  14 , which are longitudinally moveable relative to tongue rails  21 , which merge into control rails  16 ,  18 , and of which a tip region  20  and a heel region  22  are illustrated in sectional views in  FIGS. 3 and 4 . In order to allow a relative movement between the stock rails  12 ,  14  and the tongue rails  21  merging into the control rails  16 ,  18 —in dependence on the ambient temperature—the stock rails  12 ,  14  are directed outwards away from the tongue rails  21 , so that a continuous transition from the tongue rails  21  or their tips  20  onto the stock rails  12 ,  14  can be ensured regardless of the effective length of the stock rails  12 ,  14 . 
     The control rails  16 ,  18  consist of rail steel. Since the tongue rails  21  are subject to particularly high wear, they may in accordance to the invention&#39;s teaching consist of hard-to-weld or non-weldable materials, in particular high-tensile steel, since the tongue rails  21  merge into the control rails  16 ,  18  in a positive and non-positive manner without any welding joints. For this purpose are provided so-called tongue adapters, which are to be referred to as holding devices  24  and via which the tongue rails via their heel regions  22  merge flush directly or indirectly into the control rails  16 ,  18 . In this it is not absolutely necessary for the control rail to be connected directly to the tongue rail  21 . Rather, the adapter can possess the geometry of a section of the control rail in its track region and be joined, e.g. welded, by its end face to the control rail  16 ,  18  extending outside of the adapter. 
     Inside the holding device  24 , the tongue rail end (region  22 ) and a section of the control rail  16 ,  18  should merge together in an oblique joint  25  of in particular 30°. 
     The adapter is embodied in a block-like manner and possesses a channel-like recess  26 , which extends along the longitudinal rail direction and into which the root-sided region  22  of the tongue rail  21  can be inserted. Next to the tongue rail section  22  a wedge element such as a wedge anchor plate  28  is subsequently inserted into the channel-like recess  26 . The wedge anchor plate  28  is secured by means of one or preferably several bolts  30 , which engage with a nut or a head  32  into a T-slot  34  in the adapter, which facilitates simple tightening and securing. 
     The holding device  24  or the adapter itself in this embodiment example rests upon the foot  36  of the stock rail  16  and is joined to the latter via support wedges  38  by in particular welding. The foot  36  in turn originates from a baseplate  40  and preferably is welded to the latter. 
     On both sides of the holding device  24 , which as mentioned above can be referred to as adapter, extend ledge-like walls  42 ,  44  that are connected, e.g. welded, to the baseplate  40 . Further, support elements may originate from the baseplate  40  on the exterior side, as is purely schematically illustrated by the support element  46  in  FIG. 4 . 
     The right end wall  42  shown in the illustration of  FIG. 4  possesses in its head region a flat steel element  48 , which extends along the direction of the tongue rail  21  or its root section  22  and serves in the function of a side rail. The flat steel element  48  is welded to the end wall  42 . 
     The region of the adapter or holding device  24  is for the most part contained in a housing, which gives rise to assembly boxes  52 , which are composed of the walls  42 ,  44  as side walls, the ledge  48  as well as a plate-shaped flat element  54  originating from the upper end face of the wall  44 . 
     As is illustrated in  FIG. 1 , the elements  48 ,  54 , which also can be referred to as ledge-like are also bolted or welded to the walls  42 ,  44 . 
     Outside of the adapter or holding device  24 , a side rail  50  extends along the rail expansion that may merge into the wall  42 . In this, the side rail  50  originates from the baseplate  40 . 
     Independent hereof, it is not necessary for the baseplate  40  to extend continuously from the start to the end of the rail expansion. Rather, several baseplate sections may be provided, which are separated by some distance and which are connected by the side rail  50  or the wall  42 . 
     Within the holding device  24 , the tongue rail  21  is supported by the bottom surface  56  of the channel-like recess  26 . Wedging the tongue rail  21  results in both horizontal and vertical immobilization. Outside of the holding device  24 , the tongue rail  21  is supported on a guide rail  58 , which originates from the side rail  50 . In this, the guide rail  58 , which can also be referred to as slide plate, extends—with its longitudinal edge that extends on the side-rail side—into a geometrically matching groove- or slit-shaped receptacle  60  of the side rail  50 . The guide rail  58  at the same time serves as lateral support of the stock rail  12 , which on the outer side, i.e. the opposite side, is supported by a second guide in form of a vertically extending plate element  62 . Consequently, the stock rail  12  is able to move along the longitudinal direction between the guide rail  58  and the guide plate  62 , which also can be referred to as guide plate, i.e. is able to expand and contract. A horizontal immobilization is achieved by laterally restricting the stock rail  12  in between the guide rail  58  and the guide plate  62 . A vertical movement is prevented by the fact that the stock rail  12  with a longitudinal foot edge region  64  engages into a geometrically matching cut-out  66  of the guide plate  62 . In this, the recess or cut-out  64  is bordered by the baseplate  40  on the ground side. 
     The rail tongue  21  rests upon the guide rail  58 . Horizontal immobilization or immobilization to a large extent is achieved by arranging spacer strips  72  between the outer side  68  extending on the side-rail side and the facing inner surface  70  of the side rail  50 , whereby the spacer strips  72  preferably are detachably mounted to the guide rail  58  by means of screwed connections. Horizontal movement of the tongue rail  21  is ruled out by the feature that the tongue rail  21  undercuts the head  74  of the stock rail  12 . 
     The illustration of  FIG. 3  further shows that the guide rail  58  may be supported by plate-like support elements  76  that originate from the baseplate  40 . Furthermore, the guide plate  62  is secured on the outer side via support elements  78 . 
       FIG. 5  shows a sectional view along the line D-D of  FIG. 1 , which is situated in front of the holding device  24  or the assembly box  52 . Discernable is the side rail  50 , from which originates the guide rail  58 , upon which rests a section  23  of the tongue rail  21 . With the side of its web facing the tongue rail, the stock rail  12  is in contact with the guide rail  58 . On the opposite side, the guide plate  62  extends along the longitudinal direction of the stock rail  12 . Also illustrated is a plate element  76  that supports the guide rail  58 . 
     As shown in  FIG. 2 , a drainage box  80  is arranged at the end of the rail expansion device  10 , i.e. in front of the tongue tip in the drawing. The drainage box  80  is connected to the interior spaces of the rail expansion device  10  that are contained between the stock rail  12  and the guide plate  62 , and the side rail  50  and an exterior enclosure  80 . There also exists a connection between the areas bordered by the support elements  76  and the above-described exterior chambers. In addition, a shorting connector  84 —i.e. a power cable with large cross-section—extends immediately adjacent to the stock rail  12 , i.e. in the exterior chamber bordered by the guide plate  62  and the stock rail  12 . The shorting connector guarantees electrical conductivity between the stock rail  12  and the control rail  16 .

Summary:
Rail expansion device having stock and tongue rails which can be displaced longitudinally with respect to one another and bear against one another transversely with respect to the longitudinal direction of the rails under the application of force, wherein a holder releasably holds the tongue rail at a distance from the region thereof which bears against the stock rail, and wherein the tongue rail in the holder is non-positively and positively connected to a standard rail passing into it or to a section thereof or to a section of the holder having a profile which merges with that of the standard rail.