Alpine, I recollect reading there was a wye at Hancock, which was extended for the Rotary, so helpers from the west would have to go through the Tunnel down to Hancock and then back through the Tunnel again to get home without a Turntable.
I do not recollect reading about when the Boreas Pass Engine House was built but stone makes a lot of sense, they had adequate supplies to hand and used stonemasons anyway. A wooden structure would have had to have been massive to resist the weather. Boreas was also a small town at one time, now quite why I am not so sure, perhaps more in the summer. It had a Post Office.
There is mention of snow sheds being moved to Boreas from Alpine as they were not up to dealing with the slides, but snow sheds seem to have a limited life expectancy. When they closed Boreas in 1910 they were considered in normal condition, for snow sheds, but not worthy of taking down for their materials.
Boreas had engine facilities at the bottom of both sides.
Now why they replaced with a wye when the Engine Shed at Boreas burnt is is something I have not seen mentioned, I assume it was cheaper and they were not looking long term. Alpine maybe because by then it was realised it was a seasonal operation?
In later years the helpers would run quite long distances, presumably there was not enough business to warrant the expense of additional facilities. One of the steepest grades was over Kenosha and initially they had facilities at Grant and then Webster but nothing nearer than Como on the other side. There was a wye on Kenosha but no snow sheds, however that spot does not seem to attract snow drifts.