Here we have a basic oval of track. Standard HO switches allow a long passing siding and small spur. Note that the spur would be useful if you chose to add an extension to this layout. This type layout is popular with those who enjoy simple, straightforward operations.
It only takes two switches to make a lower spur siding and a second siding extending from the top. These can be used as sidings. They also make extending the layout easier, should you add to the left or bottom.
More switches can do all this. Our sidings are now connected on the lower left, and we have a new spur on the lower right. Inside the original oval are two sets of sidings. You can easily add an extension to the bottom, right or left as well as enjoy more continuous running and switching work.