Two problems about interface and refined grid
 

I'm a newbie for Fluent. I face two problems:

1: I want to calculate the passing of two trains with non-conformal grid interface. The two grids is merged into one by Tgrid. The grid succeeds in passing grid check but fails to construct interface with a prompt "Error: Compute-node-normal: degenerate faces." I do not know why?

2: I use Gridgen to generate grids. I have found the grids with small wall distance can succeed in passing the check of Gridgen with all positive volume and area, but fail to passing Fluent's grid check with negative volume. All comments are warmly welcomed! Thank you, very much!