4 and the lower bottom support mechanism 5 are located in an area enclosed by the four standing posts 1. The second fixing member 102 on a post body of each of the four standing posts 1 is hinged to the upper bottom support mechanism 4. The lower bottom support mechanism 5 forms a second decker of wagon structure. The lower bottom support mechanism 5 is hinged to the third fixing member 103 on the post body of each of the four standing posts 1. In a preferred implementation, the first fixing member 101 can be provided at an upper end or another position of the standing post 1, and the third fixing member 103 can be provided at a lower end or another position of the standing post 1, as long as the first fixing member 101, the second fixing member 102, and the third fixing member 103 are sequentially arranged from top to bottom. There is no side enclosure structure provided at a side of the second decker of wagon structure, which facilitates the placement of irregularly sized or large-sized items. Alternatively, a cloth bag can be provided to store small items.

The upper bottom support mechanism 4 includes a first crossing member and a second crossing member that are mutually hinged. The first crossing member is hinged to the second fixing members 102 on first standing post 11 and fourth standing post 14 of the four standing posts 1, and the second crossing member is hinged to the second fixing members 102 on second standing post 12 and third standing post 13 of the four standing posts 1. The upper bottom support mechanism 4 uses double supporting members, increasing the support strength of the upper bottom support mechanism 4.

In Embodiment 1, as shown in FIGS. 2 and 3, the first crossing member and the second crossing member each include first connector 40, as well as first connecting tube 41, second connecting tube 42, third connecting tube 43, and fourth connecting tube 44 that are rotationally connected to the first connector 40 at a limited angle. The first connector 40 is provided with four connecting arms respectively facing the four standing posts 1. The connecting arms each are provided with a clamping slot with a horizontal bottom and a notch facing downwards. Referring to FIGS. 2, 3, 6, and 7, when the wagon switches from the folded state to the unfolded state, the connecting tube rotates to the corresponding slot. Due to the clamping slot, the connecting tube cannot continue to rotate and is in a horizontal state. The first connecting tube 41 and the second connecting tube 42 of the first crossing member are respectively rotationally connected to the second fixing members 102 on the post bodies of two standing posts 1 at a limited angle. The first connecting tube 41 and the second connecting tube 42 of the second crossing member are respectively rotationally connected to the second fixing members 102 on the post bodies of the other two standing posts 1 at a limited angle. The third connecting tube 43 and the fourth connecting tube 44 of the first crossing member are respectively rotationally connected to the third connecting tube 43 and the fourth connecting tube 44 of the second crossing member at a limited angle in a sliding manner through sliding sleeve member 72 provided on suspension rod 71. The standing post 1 is provided with a mounting arm for mounting the connecting tube. The mounting arm is provided with a clamping slot with a horizontal bottom and a notch facing upwards. Limited by the clamping slot, the connecting tube cannot continue to rotate and is in a horizontal state. When the wagon is folded, the first connector 40 rises, and the first connecting tube 41, the second connecting tube 42, the third connecting tube 43, and the fourth connecting tube 44 rotate downwards around a connection point on the first connector 40 as a center of rotation and cluster around the first connector 40 as a center.

In Embodiment 2, the upper bottom support mechanism 4 includes two or more crossing members that are sequentially hinged. The connection between the crossing member at each end and the corresponding standing posts 1 is the same as that in case there are only two crossing members.

If the wagon is a large-sized one, it will inevitably increase the size of the upper bottom support mechanism 4. In the present application, the upper bottom support mechanism 4 includes two or more supporting members, shortening the length of the connecting tube. If the upper bottom support mechanism 4 includes only one supporting member, the connecting tube will be too long, causing the horizontal height of the first connector 40 after folding to be greater than the top horizontal height of the standing post 1, thereby increasing the height of the wagon after folding.

As shown in FIGS. 2 and 4, the double-decker wagon further includes a pair of connecting mechanisms 7 that are respectively located at front and rear sides. The connecting mechanisms 7 each include the suspension rod 71, the sliding member 72, and limiting member 73. The sliding member 72 is provided on the suspension rod 71 in a sliding manner, and the limiting member 73 is provided at a first end of the suspension rod 71 to prevent the sliding member 72 from slipping off from the first end of the suspension rod 71. A second end of the suspension rod 71 is hinged to a middle hinge point of the side folding mechanism 2 at a same side. The third connecting tubes 43 of the first crossing member and the second crossing member each are rotationally connected to the adjacent sliding member 72 at a limited angle, while the fourth connecting tubes 44 of the first crossing member and the second crossing member each are rotationally connected to the adjacent sliding member 72 at a limited angle. For the connection form, please refer to the connection form between the standing post 1 and the first connecting tube 41, which will not be repeated herein. When the wagon is in the unfolded state, the sliding member 72 abuts against the limiting member 73, and the suspension rod 71 assists in supporting the upper bottom support mechanism 4, further increasing the strength of the upper bottom support mechanism 4.

As shown in FIGS. 1 to 3, the lower bottom support mechanism 5 includes second connector 50 and four fifth connecting tubes 51 that are rotationally connected to the second connector 50 at a limited angle. The four fifth connecting tubes 51 are respectively rotationally connected to the third fixing members 103 on the four standing posts 1 at a limited angle. For the connection form, please refer to the connection form between the first connecting tube 41 and the standing post 1, which will not be repeated herein. When the wagon is folded, the second connector 50 rises, and the four fifth connecting tubes 51 rotate downwards around a connection point on the second connector 50 as a center of rotation, and the four fifth connecting tubes 51 cluster around the second connector 50. In other implementations, the lower bottom support mechanism 5 can also include two or more supporting members. In the present application, taking into account the folded volume, weight, support strength, and unfolded volume of the wagon, the upper bottom support mechanism 4 preferably includes two supporting members, and the lower bottom support mechanism 5 preferably includes one supporting member.

As shown in FIGS. 1 to 4, the double-decker wagon further includes rear frame mechanism 3. One end of the rear frame mechanism 3 is rotationally connected to lower ends of the second standing post 12 and the third standing post 13 at a limited angle. The rear frame mechanism 3 can only rotate from a vertical state to a near horizontal state to expand the lower bottom support mechanism 5, making it easy to pick and place large items. The rear frame mechanism 3 includes first rear frame standing post 31, second rear frame standing post 32, and rear frame folding assembly 33. The rear frame folding assembly 33 is connected between the first rear frame standing post 31 and the second rear frame standing post 32. When the double-decker wagon is folded and unfolded, the rear frame folding assembly 33 is correspondingly folded and unfolded to reduce the folded volume of the rear frame mechanism 3. A first end of the first rear frame standing post 31 is rotationally connected to a lower end of the third standing post 13 at a limited angle, while a first end of the second rear frame standing post 32 is rotationally connected to a lower end of the second standing post 12 at a limited angle. Specifically, the lower ends of the second standing post 12 and the third standing post 13 each are provided with a limiting arm. The limiting arm is provided with a limiting groove that is open upward. The limiting groove runs through the entire limiting arm, and a bottom of the limiting groove is close to horizontal. The first ends of the first rear frame standing post 31 and the second rear frame standing post 32 are respectively rotationally connected to the limiting grooves of the third standing post 13 and the second standing post 12 through a rotating shaft. When the first rear frame standing post 31 and the second rear frame standing post 32 are unfolded and clamped into the limiting grooves, a rod body at the first end abuts against a bottom of the groove and cannot continue to rotate. In this way, the first rear frame standing post 31 and the second rear frame standing post 32 have supporting force and can hold items, expanding the support area of the upper bottom support mechanism 4.

As shown in FIGS. 1 to 4, the first rear frame standing post 31 and the second rear frame standing post 32 each are provided with fold-down sliding sleeve 34 in a sliding manner. The fold-down sliding sleeve 34 is slidable on the standing post 1. Second ends of the first rear frame standing post 31 and the second rear frame standing post 32 each are provided with fold-down fixing member 35. The fold-down fixing member 35 and the fold-down sliding sleeve 34 on the first rear frame standing post 31 are respectively connected to first ends of the rear frame folding assembly 33, and the fold-down fixing member 35 and the fold-down sliding sleeve 34 on the second rear frame standing post 32 are respectively connected to second ends of the rear frame folding assembly 33. The fold-down sliding sleeve 34 is provided with clamping assembly 36. The clamping assembly 36 includes a connecting part and a clamping part that are connected to each other. The connecting part is connected to the fold-down sliding sleeve 34. The clamping part is provided with a clamping slot, with a notch narrower than a middle part. Referring to FIGS. 6 and 7, when the fold-down mechanism is in a storage state, the clamping parts on the first rear frame standing post 31 and the second rear frame standing post 32 are respectively clamped to the rod bodies of the second standing post 12 and the third standing post 13, and the clamping parts remain stably connected to the corresponding standing posts 1 without external force. In a preferred implementation, the connecting part, the clamping part, and the fold-down sliding sleeve 34 are integrally formed, and are arranged on the corresponding standing post 1 in a sliding manner through the fold-down sliding sleeve 34. Specifically, the rear frame folding assembly 33 includes first crossing bent tube 331, second crossing bent tube 332, first crossing straight tube 333, and second crossing straight tube 334. Middle parts of tube bodies of the first crossing bent tube 331 and the second crossing bent tube 332 are rotationally connected through a rotating shaft. A first end of the first crossing bent tube 331 is rotationally connected to the fold-down fixing member 35 on the first rear frame standing post 31 through a rotating shaft. A first end of the second crossing bent tube 332 is rotationally connected to the fold-down sliding sleeve 34 on the first rear frame standing post 31 through a rotating shaft. Middle parts of tube bodies of the first crossing straight tube 333 and the second crossing straight tube 334 are rotationally connected through a rotating shaft. A first end of the first crossing straight tube 333 is rotationally connected to the fold-down fixing member 35 on the second rear frame standing post 32 through a rotating shaft, and a first end of the second crossing straight tube 334 is rotationally connected to the fold-down sliding sleeve 34 on the second rear frame standing post 32 through a rotating shaft. A second end of the first crossing bent tube 331 is rotationally connected to a second end of the first crossing straight tube 333 through a rotating shaft, and a second end of the second crossing bent tube 332 is rotationally connected to a second end of the second crossing straight tube 334 through a rotating shaft. When the double-decker wagon is folded, the rear frame folding assembly 33 is folded synchronously. At this point, the first rear frame standing post 31 and the second rear frame standing post 32 are close to each other, and the fold-down sliding sleeves 34 slide downwards. The first crossing bent tube 331, the second crossing bent tube 332, the first crossing straight tube 333, and the second crossing straight tube 334 cross and rotate to a limit position. Limited by the rear frame folding assembly 33, the fold-down sliding sleeves 34 cannot rotate when sliding along the standing posts 1, ensuring that the clamping slots of the clamping parts always face the second standing post 12 and the third standing post 13.

Referring to FIG. 2, the four standing posts 1 include the first standing post 11, the second standing post 12, the third standing post 13, and the fourth standing post 14. First side folding mechanism 21 is hinged between the first standing post 11 and the second standing post 12. Second side folding mechanism 22 is hinged between the third standing post 13 and the fourth standing post 14. Third side folding mechanism 23 is hinged between the first standing post 11 and the fourth standing post 14. Fourth side folding mechanism 24 is hinged between the second standing post 12 and the third standing post 13. The first side folding mechanism 21 and the second side folding mechanism 22 each include a third crossing member, a fourth crossing member, and a fifth crossing member that are sequentially hinged. In other implementations, they may also include more than three sequentially hinged crossing members. The third side folding mechanism