Our basic oval of O27 track
We add another oval around it. Note that this oval uses 42" curves.
The switches in the upper left-hand corner are O27. They connect the two loops. The pair on the right are 42" switches made for O27 track. You can add one pair at a time. The result is a two-train operation that can even handle three trains if you use blocks on the outer loop.
A reversing siding within the inside loop adds action, thanks to a pair of switches and an extra straight (the other two straights are the ones replaced by the switches).
One switch at a time, you can add sidings for small freight drills and operating accessories. This layout is great for someone who likes using the various loaders and other accessories. Each of the four additional switches makes a siding, so you can expand one siding at a time.
The three switches added to the inner loop area are O27, the one added to the 42" loop is a 42" switch.