The reference to trains passing at Alna Center was made in connection with turning trains back at Top of the Mountain, which may be the operating routine for some time. Actually, except on special occasions, I don't think there will be much need to pass trains at Alna Center, even after we're running to Route 218.
And concerning spring switches, at the WK&S near Allentown, Pa., where I hang out when not in Maine, we have a crossover with two spring switches, which we use to run around the train at Kempton. It works just dandy.
If we have a single-end siding at Top of the Mountain, we'd need about 180 feet of track for clearance on the siding, and 120 feet or so on the main track. If we're going to park the tank car up there, that still leaves 150 feet, plenty of space for three passenger cars or four freight cars. I would think we'd only park the tank car up there during times of greatest fire danger.