WO 00/10859 PCT/SE99/01399 5 A structural element arranged to form at least a part of a shell of a car body of a railway vehicle TECHNICAL FIELD OF THE INVENTION AND PRIOR ART 10 The present invention relates to a structural element arranged to form at least a part of a shell of a car body of a railway vehicle, wherein the structural element comprises an external layer comprising a metal material, an intermediate layer arranged to 15 form a thermal insulating layer and an internal layer. In conventional manufacturing of a side wall of a car body of a railway vehicle, an external metal sheet which is provided on internal beams in order to increase the stiffness of the metal 20 sheet is used. In order to prevent buckles to occur in the sheet, supporting elements are provided which are connected to the sheet by means of welding. Furthermore, a ventilation layer is provided in connection with the sheet and a thermal insulating layer and finally an internal panel. Such a side wall for a car 25 body is expensive and complicated to manufacture and it has a large thickness, which encroaches on the internal volume of the car body. Furthermore, irregularities easily occur in the sheet as a consequence of internal treatment by, for example, welding during the mounting of said supporting elements to the sheet. 30 From DE 38 38 686, EP 544 473 and DE 195 33 532 structural elements are known, which are intended to be used for forming shells of a car body of a railway vehicle. The structural elements in these documents comprise, in addition to the initially 35 mentioned three layers, also reinforcement elements in form of through beams or surrounding beams. Such reinforcement WO 00/10859 PCT/SE99/01399 2 elements have the effect that the structural elements obtain a relatively complicated construction, which increases their manufacturing cost. Such reinforcement beams also increase the weight of the structural elements, which makes its handling 5 more difficult. DE 296 08 643 U1 shows a structural element which comprises a portion of a side wall, a roof and a floor of a railway vehicle. The structural element comprises external and internal layers 10 formed by a fibre reinforced plastic material. Said external and internal layers are connected to each other along the edges of the structural element, wherein they enclose an intermediate layer. This structural element comprises longitudinal reinforcement beams at the roof and floor portion of the 15 structural element. Such reinforcement beams lead also in this case to a relatively complicated construction of the structural element. Furthermore, it may be difficult to obtain a completely even and smooth external surface in a fibre reinforced plastic material. 20 SUMMARY OF THE INVENTION The object of the present invention is to provide a structural element of the initially mentioned kind, which is inexpensive to 25 manufacture and which has external and internal surfaces of high quality at the same time as it enables a rapid and uncomplicated mounting in the vehicle. This object in achieved by the initially mentioned structural 30 element, which is characterised in that the intermediate layer is fixedly connected to the external layer and the internal layer so that a structural element in one piece is formed, wherein the structural element is arranged to constitute a supporting element of said vehicle body. By such a design, the structural element 35 does not require through beams or surrounding reinforcement beams, wherein the manufacturing of the structural element is WO 00/10859 PCT/SE99/01399 3 considerably simplified. Such a structural element may also be manufactured to a low cost at the same time as it may be made essentially lighter with respect to weight than by prior art technique. Very advantageous is the fact that, the structural 5 element comprises an intermediate layer of a thermal insulating material. Thereby is obtained that no further thermal insulation need to be provided internally in the vehicle. A wall construction may thereby be manufactured essentially thinner than by a conventional construction and encroaches thereby less on the 10 internal volume of the vehicle. According to a preferred embodiment of the invention, also the internal layer comprises a metal material. Such a metal material may be steel or aluminium. Preferably, both the external and 15 internal layer comprise sheets of metal which in co-operation with the intermediate layer forms a structural element with such a stiffness that it may constitute a supporting component of a vehicle body. Alternatively only the external layer may comprise a metal material. Thereby, the external layer will be 20 dimensioned so that it will carry the main load of the structural element but it will at the same time be propped and guyed by the remaining layers so that the structural element without problem may be used as a supporting component. 25 According to preferred embodiment of the invention, said intermediate layer is essentially thicker than said internal and external layers. In order to provide stability to the external and internal layers, the intermediate layer comprises such a stiff material that it supplies stiffness to said structural element. 30 Such a stiff intermediate layer prevents also in an effective way that buckles and deformations occur in the relatively thin external and internal layers. Consequently, such an intermediate layer has two functions, namely to stiffen the external and internal layers as well as to provide thermal insulation. 35 WO 00/10859 PCT/SE99/01399 4 According to preferred embodiment of the invention, said intermediate layer is fixedly connected to the internal and external layers by means of a binder means arranged to work so that said layers are connected along essentially its entire 5 surface. By connecting said layers to each other along essentially the entire surface, it is prevented that a local displacement between the layers occurs under load. Thereby, the structural element will constitute a very stable, stiff unit which distributes applied loads essentially along its whole 10 construction. Said binder means may consist of an adhesive having good adhesive properties. Hereby, the external and internal layers will obtain a very even and fine surface, which has good strength against buckles and deformations. 15 According to preferred embodiment of the invention, the external and internal layers of said structural element comprise at least a curved portion. Usually, it is desired that a shell of a vehicle is manufactured with curved portions in order to, for example, decrease the air resistance of the vehicle and to use available 20 loading profile to obtain an aesthetically attractive visual sensation. A structural element according to the present invention, which does not comprise stiffening beams, may in a rather uncomplicated way be manufactured with curved surface portions but may also be performed with a plane surface. Said 25 relatively thin external and internal layers may without problem each be bent to a desired shape individually or during the joining process with the intermediate layer. The intermediate layer may with a suitable choice of material also without problem be manufactured in a desired shape, wherein such a material 30 may be a plastic material. Thereafter, said layers are connected with a suitable adhesive in order to form a structural element, which has desired curved surfaces. According to a preferred embodiment of the invention, the 35 structural element comprises fastening devices for equipment in the vehicle. Since the internal layer of the structural element has WO 00/10859 PCT/SE99/01399 5 a flat surface, which also may carry loads, the possibility arises to fasten in the vehicle provided internal equipment in the structural element. For example, fasting channels for seats may be provided in the structural element, such channels may be 5 fastened in the internal layer or provided so that they also extend into the intermediate layer. Further material savings are thereby obtained since there is no need for supporting elements, for such equipment. 10 According to another preferred embodiment of the invention, said structural element forms at least a portion of a side wall or a portion of a roof of a car body of a railway vehicle. The present structural element is very favourable to use in railway vehicles such as in underground railway cars and in railway cars 15 for transportation of people. It is in the first place the side walls of the car body, which are suitable to be comprised by said structural element. By making the walls thinner a greater internal volume for the accommodation of passengers may be obtained. The vehicle car will also be essentially lighter, which 20 fact, i.a., influences the energy consumption in a favourable way. It also favorable to use the present structural element in the roof of the car body, which by this measure obtains a light and uncomplicated construction. To use the structural element in a floor construction of a railway vehicle is also an alternative, 25 which may not be excluded. BRIEF DESCRIPTION OF THE DRAWINGS In the following, preferred embodiments of the invention are 30 described as examples with reference to the attached drawings, in which: Fig 1 shows a plane view of a car body of a railway vehicle comprising a structural element according to the 35 present invention, WO 00/10859 PCT/SE99/01399 6 Fig 2 shows schematically a cross sectional view of a structural element according to the present invention. DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS OF 5 THE INVENTION Fig 1 shows a car body 1 of a railway vehicle, comprising a structural element 2 according to the present invention. The structural element 2 forms at least a part of a side wall of the 10 car body 1 and has openings for windows 3. The structural element 2 may, for example, extend between two door openings 4, of which one door opening 4 is shown in Fig 1. The structural element 2 comprises an external layer 5 of a first piece of material comprising a metal material, an intermediate layer 6 of 15 another second piece of material comprising a thermal insulating material, and an internal layer 7 of a third piece of material. The intermediate layer 6 is arranged to be joined together with the external 5 and the internal 7 layers by means of an adhesive so that the three pieces of material form a structural element 2 in 20 one piece. This structural element 2 is intended to be mounted on the vehicle by means of fastening means, which may be applied to suitable portions of the structural element 2. The fastening means, which may, for example, be rivet joint connections or joint bolt connections, may thereby connect the 25 structural element 2 at the top to the roof of the railway vehicle and at the bottom to a lower frame 9. Possibly, also fastening means may be applied for connecting the structural element 2 to adjacent side walls. 30 Before the mounting of the structural element 2 in the vehicle, fastening devices for internal equipment may be attached to the structural element 2. Such a mounting of, for example, interior details is considerably simpler to perform before the structural element is mounted to the vehicle since a mounting inside the 35 vehicle always leads to a limited mounting space. Equipment, which may be fixed to the structural element 2 internally, is, for WO 00/10859 PCT/SE99/01399 7 example, fastening channels for seats and sound absorbing panels. Fig 2 shows schematically a cross sectional view of a structural 5 element 2 according to the present invention. The structural element 2 thus comprises an external layer 5, an intermediate layer 6 and an internal layer 7. Preferably, the external 5 and internal 7 layers consist of metal sheets of, for example, steel or aluminium, which layers essentially supply the supporting 10 properties to the structural element 2. However, it is possible to manufacture the internal 7 layer in other materials such as, for example, fibre reinforced plastic materials. The internal 7 layer may also be performed in a more pliant material, such as plywood, under the condition that the external 5 layer is 15 manufactured in an essentially stiffer material, such as, for example, steel. Preferably, the intermediate layer 6 consists of a stiff, thermal insulating material. Such a material may comprise a foam plastic material of PVC, PMI, PEI or PUR. Alternative materials for the intermediate layer 6 are balsa wood and other 20 materials having good thermal insulating properties. Preferably, such thermal insulating materials may be formed in a honey comb structure. The intermediate layer 6, which is essentially thicker than the external 5 and internal 7 layers, is arranged to provide both stiffness and thermal and sound insulating 25 properties to the structural element 2. At the edges of the structural element 2, the material of intermediate layer 6 may be in full view or covered by, for example, a plastic material 10, in order to protect the material of 30 the intermediate layer 6 against, for example, damp. The present invention is not in any way restricted to the described embodiments but may be varied and modified within the scopes of the claims. 35