I had asked this question before and maybe nobody knows. To make my plan work I need a half circle of O-64 in the yard. The mainlines will be O-72 and O-84. Will a Lionel Challenger and a Williams Niagara traverse the O-64 at low speeds? I cannot design around it and do what I want to do. Thoughts?
I don't think you'll have any issue with the Williams Niagara, but the Challenger may be more of a "challenge". I know my VL-BB with a similar tender won't make O60 Fastrack curves. The locomotive makes it around, the the tender derails with all the in-line wheels. The Challenger appears to have a similar design tender with the 4-wheel truck and then the ten wheels fixed in-line.
I have found that when a manufacturer says minimum radius 0-72 they mean it. I have several Lionel articulated locomotives and a few brass 3rd Rail that are O-72 so I built my layout with a minimum radius of O-72~