My guess it that the offending diagonal rails were formed with n-gon sides; that would have a concave angle on the bottom, and Poser will display a concave n-gon by drawing a straight line from the bottom of one end to the bottom of the other end. So, for example, a Star of David n-gon will show as a hexagon.

Try exporting as an OBJ, if that option is available. The mesh must be UV-mapped to accept image map textures.

Poser 12, in feet.  

OSes:  Win7Prox64, Win7Ultx64

Silo Pro 2.5.6 64bit, Vue Infinite 2014.7, Genetica 4.0 Studio, UV Mapper Pro, UV Layout Pro, PhotoImpact X3, GIF Animator 5