Side view of railer in use.
The 2mm black guide that I have used seems to clear the underside of stock OK, although the horn of the trailing Kadee coupler drags very slightly. However I have tested this with a 6-axle diesel and even then it's not enough to matter. Using a quite shallow angle of descent helps a lot with this.
However stock with a long overhang may cause problems and a thinner guide would help, although wheel guidance would be less positive. Rather I would either use two pieces of plastic as guides with a gap in between, or to modify this one just mill a slot in the problem area.