^^Sometimes you assume right, sometimes you assume wrong. I still choose the most simple, and logical, way and ask the person that already knew the answere. No harm done. Can we please move on with the topic now?
UC said:^They do this if the middle isn't supported.
Otherwise, you end up with a very long cantilever section that isn't stable if not supported from both ends.
Basically, you build it up to (or just a little past) the closest support. If you look at most B&M sections, they're supported near the connection on one end of the track, and the next piece is supported on the connection to the next piece, etc.