Not sure I understood. Perhaps the only UP operation might be on the shortest safe route between East Portland and BNSF? If so, that would imply not turning it on a local wye if it's under UP's control. Would it have to be moved dead, so there's no possibility of operating a non-UP steamer on UP tracks?

As for curvature, how does that compare to the curve on the new access to Old Sacramento and the State Railroad Museum trackage? Several years ago, shortly after the new connection opened, 844 almost made it through backing in - the locomotive did OK, but the tender derailed an axle. UP had to jack up and secure it to get around the curve. Everything got out OK several days later, so I presume a little adjustment of the curve happened while other affairs were going on at Old Sac.