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.
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