... the purple section shows the clearance allowance each side. Rolling the vehicle over it proves it to be correct, but it is not always the corners or mid-points of the body which create the extent, as I found earlier. On a reverse curve such as this S-transition, you get necks in the envelope which look as if they must be wrong, but on rolling the vehicle it proves to be correct. To get a reasonably smooth envelope I found that it was necessary to use very short steps: 2_270842_500000002.png The above created by drawing the dummy vehicle at 3" steps (full-size prototype!), and still it is less smooth than I would have expected. To keep memory use sensible I have limited the length of the envelope to a maximum of 5000 3" steps, i.e. 1250ft of track, or 5000mm at 4mm/ft scale. If the template is longer than that the remaining length won't have the envelope over it. It would be necessary to split it into shorter templates if required. And ...