In the European wheel arrangement, that is, at the museum in Mulhouse, France:

http://www.railpictures.net/viewphoto.php?id=157041

Did you mean 2-3-2? Or 4-6-4 in North American usage?

Yes.

I was once shown a book that had a clearer picture of this 4-6-4 than my computer screen will produce. In the picture I could see that the small tubes were finned on the inside for better heat transfer, a good illustration of how the French tried to milk their engines for maximum efficiency. I always wanted to know how these were made, and how you'd roll them. And I could see that some of the superheaters had holes burned through them, just like Type E's usually do.

How would that engine be a 2-4-2?
in *any* system?

Scot

European system is to count the wheels on one side. This engine then using the American Whytes system is a 4-6-4, In Europe it should be a
2-3-2

A friend that works for Alstrom power says that the tubes with internal surface riffled features these days are for the critical and super-critcal boilers so that the water will touch the internal tube surface and keep them from softening and bursting as there is a minute steam space between the water and the tube especially in the furnace area.

The finned internal surface of the fire tubes must have been a cinder and soot magnet.