The wheels, of course, are mounted on individual carriages, which can swivel on central pins/bearings. But on the very largest of the huge UP engines, the front carriage could also slide side-to-side. The front half of the boiler on tight curves could be seen a few feet off the centerline of the front truck! You can see it starting 11:40 here https://www.youtube.com/watch?v=fXcD6ZdPR9k
As solid and imposing if not brutal as the giant UP engines were; to enable this, there was no mechanical connection between the cylindrical boiler and the rectangular frame for perhaps the first 20 or so feet of the engine’s length. (!)
You can see this on the UP 3985 Challenger footage https://www.youtube.com/watch?v=XRZ23VVQhwQ at about 3:55 (very similar engine) and at 18:15
Of course the most amazing shots are these 70 mph UP3985 tracking shots https://www.youtube.com/watch?v=ZVcOPIaekOU
A few of the early striking looking streamlined Pennsy engines (The S-1 and the T-1, I believe) were *not* articulated and could not make some of the approach turns into the Chicago station without damaging the rails. They were retired and scrapped only a couple of years after they were introduced.
Thank you! Obviously you’re very knowledgeable.
bfl