Change of plan again. Rather than work on the outer loop (F2o-F1o) I have instead made progress today with the inner loop from B1l round to F4i.
I decided that I really needed to finish off the lower levels, so I know where I can fix the supports to carry the upper-level baseboards. This issue was stopping me from finalising the station approaches on both B1 and B5. With the lower-level track beds all in place I can now finish off the top tracks. Fingers crossed that everything will link up without overly tight curves on F1o/B1u and F5i/B5u.
The "spur to nowhere" visible at the back of B2 will eventually be the feeder for the cassette loader mentioned in a previous post. I won't do any more work on that for the moment as it would just get in the way of more important things.
The trackbed joins at B1l-F1i and F1i-F2i are interesting because here the track is not running perpendicular to the baseboard breaks. If I had made a straight trackbed join at these points, I would have had to make a diagonal cut across the tracks, which would make the alignment less stable. My solution is as follows:
The trackbed join now runs perpendicular to the track for each line, so there is a sawtooth pattern where the boards slot together. This pattern should help the boards to marry properly when being assembled. I will have to do the same on F4i-F5i as here the track will be starting to swing round to meet up with the station approach on B5u.
The "sharp" bits of the sawteeth overhang the boards they adjoin, resting on the frame of the next board. So there's now a rigid sequence to assembly and disassembly of the layout, and all future overhangs have to be consistent with that. Board F1 will now always be the first board to come out, followed by F2...F5 and then the back boards. In assembly, this sequence is reversed.
I am pleased with all of this work except for one thing: the gradient of the inner loop is much steeper than I wanted. Whereas I had planned for the incline to start on B1l as soon as the lower trackbed emerges from under the upper tracks, it actually doesn't start until halfway across F2i. So there is less distance to gain the required height, and the resulting gradient is only marginally shallower than the "standard" 6% Hornby gradient with which my trains have struggled in the past. I will look at that on my next loft visit; it may be possible to make the incline start sooner and so soften the gradient.