Shading problems on a WiP model.

[Walter]

Hey all.

I've been working on the Brown Streak train I posted a while back, and have made some progress, but I seem to have hit a huge problem.
After correctly dirt-mapping the bodyshell of the train, it started displaying this very weird shading bug.

Here's a picture:

In Zmodeler 3:

I've tried the most obvious thing already, calculating normals, but that didn't work.

Any ideas?