To move a up and down saw head down the log would be difficult, so that should rule that option out. I see no reason not to have a long track for such milling techniques. I have no proof via evidence of long track up and down sawing ( the actual remains or photos), but what we do have is long, up and down, sawn stock. The debate here is if it was one of the two types of up and down sawing and band sawn. When it comes to whip sawn stock that is much easier to distinguish due to the erratic saw marks.

Today you can go out and buy a small portable band mill with a standard track length of 20' or 24'. I did, and it didn't take long for me to realize I needed a longer track, I now have 74' and the saw moves not the log. I see the past very much the same, most mills kept the standard 20' track system, they needed 40+, 20' on either end of the head. Where is the limit? Why did they stop at 20'? Someone must of said "lets put 40' on either end of the head". Tim