What is this thing?

Is it connected to the swiveling front truck? Looks like it would help center the truck.

I believe it’s the articulated steam piping that would go to the main cylinders. I know my terminology is wrong but I think you’ll know what I mean.

After some more tinkering, I'm pretty sure it's there to prevent over-articulating of the front half of the chassis, preventing damage to the drive shaft and coupling between the two independent driven sections.

In other words, if someone tried to run it through a curve less than O72, it would bottom out against to inner wall of the boiler and force a derailment before the truck turned far enough to bust the coupling and shaft.

So it does have a real purpose and is not just for show. Guess I have to modify my smoke mount.

It's the steam pipe for the front cylinders but as you've discovered it also acts to center the boiler. Don't shorten or modify it.

On the real ones, That is pipe to front cylinders for the steam.  

The same thing is present on my copy of the C & O 2-6-6-6 Allegheny.  As others have said it replicates the steam piping for the exhaust from the front cylinders.

On the MTH Allegheny I have it does not recenter the boiler.