Scale O gauge begins to evolve in 1924 with the new voice of Modelmaker magazine drawing modelers across the nation together . Initially center 3rd rail was used ..but with assorted variations to hide the center rail with ribbon rail or smaller gauge wire . Egolf introduced 6'as a diameter for his two Pacifics
Outside 3rd rail appears by 1929 with layouts near the NYC area, where several prototypical railroads could be found to justify it's use. ( save of course steam engines )
Prior to NMRA several variations to the height and distance of the 3rd rail from the other two existed .
Outside 3rd rail requires gaps at switches ...so the outer rail jumps from side to side , hence sweeps on both sides of engines ..gaps require two sweeps ...to continual running ....at a gap the 3rd rail dips down ...so the sweep slips off or is raised up depending on direction.
Signalling was simpler wirh the outside 3rd rail operation . Blocking is simple just add a gap.
Outside 3rd rail is supported by rail chairs ...many different styles were made ...as well as a large variety of sweeps. Outside rail was commonly HO rail ...or wire or square stock...depending on your budget .
I was very fortunate to purchase a pile of outside 3rd rail,...the remains of 3 layouts ...yes with 3 different rails and a variety of rail chairs ....it has been fun picking a choosing pieces to string together to build a working layout .
So once you have a working loop,...then tbe fun of running vintage trains ...that predate the NMRA standards ...so back to back , length of sweeps etc ...all become points of excitement when crossing switches ...
All good fun ...it has been a learning curve ...I'm still learning ...