Amazing work yet again Lee. 
I have been pondering your boiler articulation problems, and unfortunately I think that I may have come up with another potential problem with a fixed length ball-joint type of connection. Have you considered the various motions that might be encountered in traversing an S-curve such as on a cross-over, or even a simple siding? Sorry for mentioning this if you have already considered it, but it seemed to be of enough significance to warrant mentioning.
