Nick Ridgway
The difference is the thickness of a piece of paper, which will be familiar to those using Templot to build and mount track and one would normally bury the template during top-ballasting.
5 inches is the depth of a wooden sleeper in 12in scale; crossing timbers are usually 6in deep, which would be 2mm in EM terms.
One is always going to think in 3 dimensions when building and laying track.