Train track part overhaul

[Philippe Hurbain]

There's a lot wrong with that part; overlapping surfaces, T-junctions.

Make sure to keep an eye on part complexity (number of triangle wise). These parts use hires primitives for tiny details where it's absolutely not necessary. In a planar area, an overlap doesn't harm, and a T-junction in orthogonal direction very rarely causes artefact. I prefer parts using these "bad" techniques if this saves a lot of triangles and don't create visual problem!