- 1 RAIL WAGON TECHNICAL FIELD [0001] The present disclosure relates to the technology of railway vehicle, in particular to a rail wagon. BACKGROUND [0002] The structure of a post used in the conventional rail wagon is shown in Figures 1 to 3, and a car body includes at least two side plates 1 arranged face to face, and a top chord 2 is arranged at an outer side of a top of each side plate 1, and multiple posts 3 are arranged on both side plates 1. The post 3 has a U-shaped cross section, and an opening side 3a fixed on the side plate 1. The post 3 is formed by three side plates welded together, and the opening side 3a of the post is welded on an outer side of the side plate 1, and a top of the post 3 is fixedly connected to the top chord 2. In a circumstance that the vehicle has a fixed total gabarite, a width of an inner portion of the car body has to be reduced when the post 3 is arranged at the outer side of the car body, which is adverse to increase the volume of the car. SUMMARY [0003] The present disclosure provides a rail wagon to overcome the defects in the prior art, which may increase a volume of a carriage and improve a supporting strength of a post for a car body. [0004]A rail wagon according to a first aspect of the present disclosure includes a car body, and the car body 7002682_1 (GHMatters) P96431.AU DENISET - 2 including at least two side plates arranged face to face, a top chord being arranged at an outer side of a top of each side plate, a plurality of posts being arranged at both side plates, and the post having a U-shaped cross section and an opening side fixed on the side plate, wherein the side plate includes an upper side plate and a lower side plate, and the upper side plate has a bottom portion fixedly connected to a top portion of the lower side plate and a top portion inclined toward an inside of the car body; the post includes an inner post and an outer post, and the outer post has an opening side fixed on an outer side of the upper side plate, and the inner post has an opening side fixed on an inner side of the lower side plate, the inner post includes an upper post and a lower post, and the upper post has a bottom portion fixedly connected to a top portion of the lower post, and an opening side matching with the opening side of the outer post. [0005] In an embodiment, the inner post and the outer post each includes a bottom surface and two side surfaces surrounding two sides of the bottom surface respectively, two side surfaces of the outer post are both of an inverted triangular shape, and two side surfaces of the upper post are both of a regular triangular shape; and edges, connected to the outer side of the upper side plate, of the two side surfaces of the outer post matches with edges, connected to an inner side of the upper side plate, of the two side surfaces of the upper post. [0006] Preferably, the side surfaces of the inner post and the side surfaces of the outer post are combined to form side surfaces of the post, each of the side surfaces of the post is of a trapezoid shape, and a top edge of the trapezoid has 7002682_1 (GHMatters) P96431.AU DENISET - 3 a length shorter than that of a bottom edge of the trapezoid. [0007] In an embodiment, the inner post and the outer post are each formed by a curved plate. [0008] In an embodiment, a bottom of the lower post is connected to a bottom surface of the car body via a surrounding plate of a concaved arc shape. [0009]In a circumstance that the wagon has a fixed gabarite, the rail wagon according to an embodiment of the present disclosure employs the post having an internal and external combination structure, which may effectively increase the volume of the wagon and improve the strength of the post. BRIEF DESCRIPTION OF THE DRAWINGS [0010] Embodiments of the present disclosure will now be described by way of example only with reference to the accompanying non-limiting figures, in which: [0011] Figure 1 is a schematic view showing a partial structure of a rail wagon in the prior art; [0012] Figure 2 is a left view of a post and a part of a car body connected to the post in Fig. 1; [0013] Figure 3 is a sectional view taken along line A-A of Fig. 2; [0014] Figure 4 is a schematic view showing a partial structure of a rail wagon according to a first embodiment; [0015] Figure 5 is a left view of a post and a part of a car body connected to the post in Fig. 4; [0016] Figure 6 is an exploded perspective view of the post according to the first embodiment; and 7002682_1 (GHMatters) P96431.AU DENISET - 4 [0017] Figure 7 is a top view of an inner post and an outer post before being welded of the rail wagon according to a second embodiment. DETAILED DESCRIPTION First embodiment [0018] As shown in Figures 4 to 6, a rail wagon according to a first embodiment of the present disclosure includes a car body, and the car body includes at least two side plates 1 arranged face to face, a top chord 2 is arranged at an outer side of a top of each side plate 1, and multiple posts 3 are arranged at both side plates 1. The post 3 has a U-shaped cross section, and an opening side fixed on the side plate 1. The side plate 1 includes an upper side plate 11 and a lower side plate 12, and the upper side plate 11 has a bottom portion fixedly connected to a top portion of the lower side plate 12 and a top portion inclined toward an inside of the car body. The post 3 includes an inner post 31 and an outer post 32, and the outer post has an opening side 32a fixed on an outer side of the upper side plate, and the inner post has an opening side fixed on an inner side of the lower side plate. The inner post 31 includes an upper post 311 and a lower post 312, the upper post 311 has a bottom portion fixedly connected to a top portion of the lower post 312, and an opening side 311a matching the opening side 32a of the outer post. [0019] In a circumstance that the wagon has a fixed gabarite, the rail wagon according to an embodiment of the present disclosure employs the post having an internal and external combination structure, that the post has one part arranged 7002682_1 (GHMatters) P96431.AU DENISET - 5 inside the car body and the other part arranged outside the car body, which may effectively increase the volume of the wagon and improve the strength of the post. [0020] In one solution of the first embodiment, referring to Fig. 6, the inner post 31 and the outer post 32 are both formed by a curved plate. The post made of an integral curved plate has no welding seam, thus is difficult to be deformed and has a higher overall strength compared to the post of the welding structure. Second embodiment [0021]As shown in Figures 5 and 7, as another implementation manner of the first embodiment, the inner post 31 and the outer post 32 each includes a bottom surface and two side surfaces surrounding two sides of the bottom surface respectively. Two side surfaces 10 of the outer post are both of an inverted triangular shape, and two side surfaces 20 of the upper post are both of a regular triangular shape. Edges, connected to the outer side of the upper side plate, of the two side surfaces 10 of the outer post matches with edges, connected to the inner side of the upper side plate, of the two side surfaces 20 of the upper post. [0022] In this embodiment, the two side surfaces 10 of the outer post and the two side surfaces 20 of the upper post for supporting the upper side plate 11 are each of a triangular shape, since triangle has a greater stability, the post having the above structure has a higher strength and a greater stability, and since the main part of the post is the inner post, the volume of the wagon won't be affected, thereby ensuring not only the volume of the carriage but also 7002682_1 (GHMatters) P96431.AU DENISET - 6 the strength and stability of the post. [0023] In a preferable solution of the second embodiment, for further enhancing the strength of the post in a height direction of the car body, the side surfaces of the inner post and the side surfaces of the outer post are combined to form side surfaces of the post. The side surface of the post is of a trapezoid shape, and a top edge of the trapezoid has a length shorter than that of a bottom edge of the trapezoid. [0024]As a preferable technical solution for all the above described embodiments, a bottom of the lower post is connected to a bottom surface of the car body via a surrounding plate 4. The surrounding plate 4 is of a concaved arc shape and is configured to prevent bulk cargos, such as ores, from being retained at the bottom of the post, thereby reducing the stress at the weld seam between the inner post and the chassis of the car body and increasing the fatigue life of the weld seam. [0025] It should be noted that, the above embodiments are only intended for describing the technical solutions of the present invention, and should not be interpreted as limitation to the present invention. Although the present invention is described in detail in conjunction with the above embodiments, it should be understood that, for those skilled in the art, modifications may be made to the technical solutions of the above embodiments, or equivalent substitutions may be made to part or all of the technical features in the technical solutions; and these modifications and substitutions also fall into the protection scope of the present invention defined by the claims. [0026] In the claims which follow and in the preceding 7002682_1 (GHMatters) P96431.AU DENISET - 7 description of the invention, except where the context requires otherwise due to express language or necessary implication, the word "comprise" or variations such as "comprises" or "comprising" is used in an inclusive sense, i.e. to specify the presence of the stated features but not to preclude the presence or addition of further features in various embodiments of the invention. [0027] It is to be understood that, if any prior art publication is referred to herein, such reference does not constitute an admission that the publication forms a part of the common general knowledge in the art, in Australia or any other country. 7002682_1 (GHMatters) P96431.AU DENISET