what is the minimal radius curve a 4-6-6-4 challenger can take comfortably
Are you talking about the Athearn Genesis or MTH Challenger? Because I know the Athearn Genesis one is rated for a minimum of a 22" radius curve. And if I remember correctly the MTH one was designed to run on (again if I remember correctly) an 18" radius curve.
If you are talking Rivarossi 18 to 22in.
Bob
I knew I was forgetting one.
the loco is a friend of mine's, i dont know the company, he wants to run it on my layout
It's probably important to know which model it is. Some of the brass challengers can't really make it around 26" curves. An old Bowser can demonstrably make it around 24". I'd guess that someone has likely made a Challenger that could get around 18" - the old Rivarossi Big Boy did that, and its rear engine isn't even articulated. (Yes, I did it. It was a 1974 edition model and at the time the largest curves that I or my friends had on a layout were 18".)