Welcome to the Forum!
I'm trying to figure out exactly what you want to do. Does your shed hold three tracks that loop around, with three going in and three going out - and your main layout somewhere else? And --- when train 'A' reaches a certain point on its track inside the shed, it stops and train 'B' starts to move?
If that's the case, it sounds like you're trying to totally automate your layout. I never met any private modelers who setup their layout that way, only commercial operations [like museums, etc.].
As for SPST: that's an abbreviation for a type of switch, Single-Pole-Single-Throw - meaning that there is one circuit ["pole"] going thru it, and it can only be 'thrown' in one way - either on or off. It's the simplest type of switch design.
Another type that is favored by model railroaders is the DPDT, or Double-Pole-Double-Throw switch. As its name implies, it has two circuits going thru it, and can get power from two different sources, with the "up" position pointing to power pack 'A', the middle position being 'off', and the "down" position pointing to power pack 'B'.
Good luck, hope that helped a little bit...
