Since I'm no railroader, I've left the entire project to Peter, and had no clear idea what he had planned. Last week, the two guys got started - and it's turned out to be a wonderful project so far. Tom, of course, perseverates on trains - but that gives him the motivation he needs to really engage for more than just a short while.
To start with, the two of them went out into our wooded backyard, and drove wooden stakes into the ground at intervals to represent the path that the railway will take.
Next, they drew a picture of the track, and added in existing features such as rocks and plants.
They then measured the distances between the stakes, using a tape measure. Tom did the measuring and called out the numbers; Peter noted them on the map.
Lastly, the added together all the measurements to come up with the perimeter of the track (the length of track to be purchased).
Unfortunately, it turns out that good quality outdoor track costs MUCH more than we anticipated! So the guys will have to practice their skills on indoor track for now - and save up (another homeschool project?!) for the outdoor system...