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This railfan is confused. I am trying to learn the various block signals on NS and am not seeing the usefulness of yellow/green/red aspect. I believe it is called Medium Approach. Seams it is redundant with a red/green/red aspect which is called Medium Clear. What does the yellow tell me if I am to follow the green route thru the plant at medium speed in either case? Is there logic to this or is it a memorize it and don't ask questions aspect rule?

Depends on where you are on NS, but yellow/green/red or yellow/green is Approach Medium in all of the former Conrail territory, which means the train must reduce to medium speed (30 mph) by the time it reaches the next signal, which would be a medium speed signal like a medium clear or a medium approach. You have to have approach medium (or approach limited in higher speed territory) in order to inform the crew that they will be taking a medium speed routing at the next signal, as you can't just go from running on clears to something like a medium clear.