The Poser box is not welded, in case anybody is wondering. You have to make your own welded box.

Same with cylinder.

Also, you may have to enable smoothing, to apply additional displacement based on welded edges.

Here I have a Poser cylinder 3 times. The second and third have been welded. All have 1 inch displacement.

The first explodes on the unwelded edge (top and bottom) but remains connected on the sides.

The second and third have crease=100.

The second no longer explodes, but has no "smoothing" so it looks wrong.

The third has smoothing, too and is closer to correct. However, as I said, Poser really can't deal with 90 degree smoothed angles too well.

But the locomotive wasn't anywhere near 90 degrees. Welding and crease angle will allow it to be displaced correctly. However, I really think the problem is that there is no displacement desired on the cylinder of the locomotive - the displacement map must be offset by .5 if gray is to mean 0 displacement.

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Renderosity forum reply notifications are wonky. If I read a follow-up in a thread, but I don't myself reply, then notifications no longer happen AT ALL on that thread. So if I seem to be ignoring a question, that's why. (Updated September 23, 2019)