Walt Rapp let me try explaining this hopefully it answers your question?
okay train is on tracks that allow the green signal light to shine bright as day. now locomotive enters the track/s that has one common rail isolated using a relay now the wheels complete a circuit allowing the relays contacts to break the green lights circuit. now the red light shines bright as day so long as any wheels are completing that red lights circuit it and only it will be lighted.
now yes the front and rear of train on the tracks not on any isolated rails is making what should be the circuit to energize the green light but remember that relays contact that moved when train entered that isolated rail section it has in fact broken the circuit allowing the green light to be lighted so yes a circuit of both is actually happening but that small contact will only allow one circuit to be energized thus only the green light or red light has a completed circuit thus rendering the other as inoperative.
I hope I helped you understand this I too am electrically challenged but have learned a few simple things others I learned to write the how to wire it up as a reference when needed another time.
and in closing I anxiously await this years Christmas layout you seem to always capture the essence of a postwar layout style thank you for sharing that.
