Dale, I believe the 4.5” comes from the distance for some Atlas switches and the space between the tracks on their dual track truss bridge. The 6” comes from those who want to play it safe for large engines like the Big Boy and long passenger cars that both overhang curves more than average. There can also be some sway, particularly at higher speeds, so while 3,5” might work in yards, it might be too close elsewhere. Whatever you go with, the only thing of importance is that there is enough clearance for the equipment you plan to run. There’s always a belief that one won’t buy large equipment, but then end up wanting too at some point in the future.
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