During use it is important that the rails extend out past the ends of the log so the cut has support the entire time.
Larger mills use a double ended bar, powered by either a single large chainsaw (typically ~120cc) and a helper handle or stinger, or two chainsaw power heads, one on either side of the mill apparatus.
This larger style of mill needs a special bar which allows the two chainsaw heads to be attached at either end.
This is no problem when cutting a single beam or large timber from a log, and would only represent significant waste if used to saw many thin boards.
Logosol is a Swedish based international company, they started making chainsaw mills in 1989.