In looking back, I remembered that it took me some time to realise that “track geometry” was the magic phrase (the one to put into google) to help me work out how to get a layout working properly. I suspect too many of us would rather leave “geometry” as a vaguely remembered school topic long since relegated for much more useful things.
As a kid I remember pulling out my trainset and fitting all
the track together from time to time, and being mystified why some curves
seemed to end up in one place and others went somewhere else. With all the
ingenuity of a 10 year old, this was seldom a problem – pushing it a bit
usually worked, after a fashion!
As a now adult helper to Al and E, this “push until it does
fit” approach no longer washes. I had to find out what was going on. It fairly
quickly became apparent that Hornby, and Tri-ang before them, make curves that
allow you to build three concentric circles (circles within circles). In fact
Hornby introduced a fourth circle not so long ago. These go by the names of first, second third
and now fourth radius. In each radius it takes eight standard curve sections to
create a circle, and if you had all four circles they would sit one inside the
other with 67 mm between them - enough clearance to allow trains to go around
them without interfering with the trains on the next circle. Hornby have a useful diagram showing how the different pieces fit together; you can find it on the Hornby website, but its not that obvious so you might like the direct link just to the diagram, which is
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