Why some of my model had a black stain??

when, i give the lighting my railroad and my cabel model had a black stain
can someone help me??
i already attch the image

Overlapping Geometry

Multiple faces in the same area, normals issue or as already said.

I’m going with overlapping geometry. Make sure you are placing them end to end and not on top of each other. You can also go into edit mode select all the verts and remove doubles the just below that recalculate (or press Ctrl+N).