I think this is pretty guirky - I thought so even as I decide to commit and do it.
Having added space and layout footage, carved from the garage and added to the basement trainroom, two mainlines were extended into the new acreage. One mainline, on the first level, did not continue its original route on the first layout space, in favor of reaching into the new room.
I decide - this is the quirky part - not to re-craft mountainside where the Mainline#1 had entered a tunnel and, instead, simply walled-up the entrance (things like that must happen in real life, right?) and continued that mainline, seen on the right foreground, here, off into the new space. Then, I added a little scenery, esp. the work-shanty, in place of the removed and newly re-directed track to ...
...suggest the walling-off had occurred some time ago.
Quirky, right?
FrankM.