Sweep & loft surface errors

Hi. I'm having problems generating some simple sweep and loft surfaces and seem unable to resolve the problem. Help please!!

Attached is part of a definition I am working on, some screen grabs of the errors I'm getting and the surface I am trying to generate drawn in Microstation.

All I'm trying to do is sweep a profile along a rail curve. The sweep surface often creates errors when the plan geometry is varied, it seems to fold back on itself. Also the facets of the surface should coincide exactly with the plan grids below, they don't.

The surface facets don't line up with the gridlines where the plan geometry arcs change in the corners, this happens both with a 1 rail and 2 rail sweep. When I create the surface in Microstation the facets line up exactly. The error (if it is an error?) is small, but enough to make the geometry pretty useless for my purposes.

As a work around I tried creating the same surface by scaling the sweep profile along the rail curve and creating a loft surface. The geometry is correct but I get random surface errors all over the place!! GGgggrrrr!!

Any help appreciated. Thank you.