This discussion has several aspects. If we're running trains with only 8 passengers, I think we're following the original WW&F model too closely.

The railcar should be the
only vehicle operating if that's often the case.
If the question is how we facilitate getting the locomotive to the south end of the train at Top of the Mountain, there are two ways to do it.
One way is to build the siding as a single end track and station no. 51 there to pull the train into the siding while the road engine rests in the clear on the main track. A load of gravel dumped at the end of main line rail will serve as a soft landing if something should start to drift away. Note: the Strasburg RR operated in this manner at Leaman Place Jct. for several years, using Plymouth no. 1 to pull trains past no. 31 to get it on the other end of the train.
The second option would be to build a switch and a temporary ramp from the siding down to meet the main track alignment to form a runaround track, with both switches on the siding elevation. When the time comes to build north from Top of the Mountain, the temporary track and switch would be removed.
Regardless, I think we need to push on to Top of the Mountain this season and next spring, since it is the logical point at which to stop for a breather. During the hiatus, repairs to the slide and washouts can be accomplished, along with construction of the Carleton Brook bridge, Sheepscot yard track changes, leach field and restrooms, etc.
At Top of the Mountain, changing directions would probably be easier than at Alna Center because the siding is on less of a grade, and the prospect of a southbound train waiting on the siding at Alna Center for a northbound to charge past is interesting.